I had this issue with 2015 and it never resolved itself and now in 2016 i still have it.
If I launch 3D warehouse from Sketchup (File>3D Warehouse>Get Models…) and try to search for anything, the window is unresponsive. Clicking on either the search button or simply pressing on the keyboard changes nothing. The mouse cursor does change from a pointer to a selection “hand”.
I can access the 3D warehouse from my browser (Chrome) and download models, and then import them into Sketchup as a workaround