Hi to all of you,
I hope you can help me 
since yesterday my model and all components are transparent? What did I do?
By the way, I did nor change the camera - I didn´t even use it!
You can look up my model, it´s a beachhouse named ck-beachhouse_2
Press k on the keyboard, it is the toggle for Show Back Edges.
wow thank you so much 
super fast and super easy - I´ld have never found out!. Thank you so much!!!
It’s a regular question, I only needed to read the title to know what the issue probably was.
I must be getting better at SU as I thought exactly the same. 
for you professionals that´s easy, but you can´t imagine how many tutorials I was watching and still have never heard about “show back edges”. I was so worried that my whole construction is destroyed 
but thanx to you help, everything is fine now 
You should probably take some time to go through the tutorials at learn.sketchup.com and watch the Square One videos on the SketchUp YouTube channel.
I was watching maybe 30 YouTube tutorials, but many of them a full professionals, which is not a big help for beginners and most of them work with SketchUp Pro Versions, which looks quite different than the Web-Version (as there is no task bar).
But yes, I started only 3 days ago and will go on learning
Go to The SketchUp YouTube Channel and pay attention to the Square One series.