I have just uninstalled an ARTX plug-in because it was not opening. When I press on Extension Warehouse button, it does not open, this means I cannot install ARTX again.
I have restarted SketchUp, then restarted my PC, but Extensions Warehouse still does not want to open.
Perhaps you used a second monitor before, and now the missing window is opening off-screen. Connect back the second monitor if it is possible.
You can also try this:
Close then Restart SketchUp.
Follow this guide exactly after you press on Extension Warehouse button:
Hold down the ALT key. Hit SPACEBAR.
Release ALT key
Hit M (Move).
Hit one of ←↑→↓ arrow keys
Move your mouse around (without pressing any mouse key) until you see the missing window.
Press left mouse button on the desired place
( Note I assume you have English windows otherwise you may need to replace “M” with a localized start letter, e.g. “L” in French)
This method may work to catch the missing plugin window as well.