Did you try to delete the privatepreferences.json file as advuised in this thread?
I did, and that didnt work for me. I did however try another option, being the ALT+SPACE > Move > arrow keys option which found the window. I have no idea how it would have ended up lost off screen but I sorted the issue and hope others are able to fix their issues too
I’m reposing this because I found it to be the easiest way to handle this kind of problem (in most cases):
It is essentially to follow exactly as I wrote, nothing more nothing less. 
- Close your SketchUp, then reopen SketchUp. Wait until fully loaded.
- Select the menu with mouse: Window>>3D Warehouse
- Hold the ⊞ Winkey and hit the →right arrow key several times until the 3D Warehouse windows appears.
The most important is to do not click with your mouse or do not hit other keys as written.
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In the other cases when the above wont work, (e.g. plugin dialog or Model Info, Preferences missing) this is more complicated, but more “sure”:
https://forums.sketchup.com/t/extension-manager-warehouse-wont-open/278207/2?u=dezmo
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fun fact, this is actually better in term of good practices and ux.
in your browser, you can open models in separate tabs, and it forces you to download separately, allowing you to open, clean and correct the models before import.
my 2cents.
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same problem have to go online directly on the warehouse internet site then download what i need. worked very well with former version of skp.