Found this thread and joined this forum because I am having the same problem as originally described. That problem being that when using the Unhide feature the item DOES NOT reappear. I’ve read through a lot of this thread and everyone points to some type of possible user error. This has happened to me more than once and IT IS NOT A USER ERROR but strongly appears to be a bug in the system. The invisible item was not accidentally deleted because you can still see its outline when referring to it in the component panel. As far as misusing layers, I never use them. My solution the this is to save often and with every save create a new version. I have a suffix on my file names Vnnn where nnn is incremented with each save. This way when something does this invisible act I can dig back through my versions and export a copy of the component to be imported into the latest version. This thread is rather old and if there is another more relevant thread please let me know.
Share your SketchUp file so we can see what you are working with. I expect there’s a simple answer that seeing your model file with make evident.
There is not a bug that erases geometry. Post you model as requested and we will sort it out.
WOW, I am really impressed with the rapid responses, this is fantastic to say the least. I put together a screen shot that better explained my situation and came to post it and another WOW there was not one but two responses from people that want to help. WOW, WOW, WOW. I will attach both the screen shot that I included notes in Photoshop and the SketchUp file. So you know the issue is not lost geometry but the fact that it is not displayed on the screen.
Workbench SketchUp 4x8 V05.004.skp (1.2 MB)
So you’ve hidden the geometry inside a component. To unhide it, you need to open the component for editing and select the geometry to unhide. As with editing any a component or a group to make other changes to the geometry, you have to be working in the context of that geometry.
This shows the hidden geometry.
And here after burrowing inside the Upper Side Beam Tenon Shoulder component and unhiding the geometry:
@DaveR is absolutely right, but managed (as usual) to reply faster than I could. Hidden works both outside-in and inside-out. That is, if a component is hidden, its contents will not be visible regardless of their state. And, conversely, if the contents are themselves hidden, the container will appear empty despite its own hidden state. And, you have to be working with the container of something open for edit to change its hidden flag. This last point often trips newbies, because Edit->Unhide All also works only within the currently open edit context.
Life would be easier without the some of the nesting. Also I would change the selected in styles to something easier to discern from locked and section lines.
I agree with RLGL. there’s excessive nesting which just creates more work and the color you chose for selected stuff is pretty close to the color for locked.
Also, there are several other objects hidden in your model. If the goal of your use of hidden is to get things out of the way while editing, you would be better served to start using layers (now called tags) to make components non-visible. Use the hidden flag only as a very temporary measure that you immediately undo as soon as you don’t need it - before closing the current context.
First thanks so much to all (DaveR, endlessfix and slaumgartner) for the QUICK responses. Still not working for me but DaveR’s response demonstrates beyond any doubt that it is something I DO NOT YET UNDERSTAND about SketchUp. So I am going to dig through, and implement, all the suggestions and come out of this knowing SketchUp better with a modeling methodology that makes me more productive.
Try this:
Go to the View menu and turn on Hidden Geometry.
With the Select tool, double click on that component with the hidden geometry inside. That’ll open the top level.
Triple click on that lower level component to open it for editing and select all of the geometry inside.
Right click on it and choose Unhide.
I followed those steps here. Unhide is at the top of the Context menu and doesn’t show in the GIF.
DaveR, sorry for taking so long to respond but spent rest of day yesterday at my wife’s Brick & Mortar classroom collecting materials to be used to teach online from home. But I did work through what you presented and it worked and I thank you. It seems needles to say but I will say it anyway, YOU PEOPLE ARE FANTASTIC! With all this though, it left me wondering how did this situation occur in the first place, after all it is not the first time it happened to me and in previous occurrences I used the Outliner panel to delete the geometry and then reconstructed. So I spent some time experimenting. First I wanted to determine if it was the complexity of my model that caused the situation. So I exported the problematic component in its parent component. If you want to follow through that component is attached. I opened a new model and imported the component. The problem was still there, so at this point complexity does not seem to be the issue. Next I again went through your suggested process right clicked on the invisible component and it provided the option to unhide it. Then because I am a curious person I right clicked on the component in the Outliner panel and the unhide option was not available. So I went back to your approach and choose unhide and it worked and was now in sync with the Outliner panel. Now in my mind the fact that the Outliner panel was not in sync with what right clicking on the component itself exposed is a synchronization situation that what I would call a “Bug” in the system unless it was supposed to work that way for some obscure reason. So I wondered more as to how did this out of sync situation occur. I played wit this super simple model hiding and unhiding, and, the component in the model stayed in sync with the Outliner panel. But then in trying several times I managed to get the out of sync situation in the super simple model. I discovered that by triple clicking on the component, Upper Side Beam Tenon Shoulder#1, then right clicking in the model and choosing hide the model became out of sync with the Outliner panel and I had to again go through your suggested process to get things back in sync. It seems the resolution for now is, do not depend on what the Outliner panel displays since it sometimes is out of sync with the actual model. So again thanks to everyone for their efforts and I now know how to fix this situation without recreating the invisible geometry. For sure without everyone’s help I would have never got this figured out. Upper Side Beam.skp (38.1 KB)
This is because you were right clicking on the component container in Outliner, not the geometry inside that component. The component container is not hidden so there’s nothing to unhide.
Again, it worked because you got inside the component container and selected the geometry in there to unhide.
What you are missing here is that Outliner shows only groups and components. It does not show loose geometry. Since what you had hidden was the loose geometry, Outliner doesn’t really come into play. There is no bug in Outliner here and no “out of sync” situation.
That’s not correct. It is never out of sync with the model. You need to understand what I’ve written. You opened the component for editing and hid the geometry inside it. The component was still visible as indicated in Outliner. You can’t see it in the model space unless you turn on Hidden Geometry in the View menu.
If you wanted to hide the component instead of the geometry inside it, you should have right clicked on the component (either in the model space or in Outliner) and hidden it. Better would learn to use layers for controlling the visibility of the components in the model space.
Here’s an analogy because I like analogies.
A component or group is a container that holds stuff. Your bottom level component is like this jar and the pasta inside is the raw geometry.
If you hide the component you are taking the whole jar with the pasta and putting it out of sight.
In your model, you opened the jar, took out the pasta, and hid it in a cupboard but you left the empty jar visible on the counter.
Thanks for your patience and detailed responses things are becoming clearer. Per your & slaumgartner’s advise I will learn how to use layers.
Good deal.
Another analogy for you because it just happened in my house. Think of Outliner as a text inventory of the objects in your model. My wife asked me to check in the fridge to see if we need milk. If I had only looked (at Outliner) I’d have said, “No. We have milk.” but we have a teenaged son so I know better than to just look. The carton was in there but it was empty. 
Formerly SketchUp ditched empty “cartons”. He (your son) may have thought that your fridge would do likewise.
Pasta, Jellybeans, Milk… all your analogies make me hungry. 
Evidently he thinks the pantry does it, too. I just found an empty cereal box.
Sorry. I’m sort of driven by food. I’m trying to find an analogy for BBQ, too. 
Not me. Just finished our Christmas dinner (here the biggest feast is on Christmas Eve). I won’t be hungry until tomorrow afternoon.