Hi,
I had the same problem. I couldn’t open or see the 3D Warehouse, Extension Warehouse, and Extension Admin windows (sorry, I have it in Spanish). I found this solution here and tried it:
Follow these steps. This is how it worked for me: Restore the SketchUp window (do not maximize or make it full-screen, and do not minimize it). Click the 3D Warehouse button and follow the steps (do not move your mouse). The key is to hold the ALT key and then hit the SPACE BAR quickly. If a small black window with a menu that includes options like restore, move, maximize, minimize, etc., appears (not in the upper left corner of the SketchUp window), you have caught the hidden 3D Warehouse window. Close the 3D Warehouse window and save the file you opened to do this. For the Extension windows, I had to do this several times until I “caught” the window.
I have two monitors. One of my three attempts worked with both monitors on. For the other attempts, I unplugged one monitor. If SketchUp crashes before saving, you’ll have to repeat the process.