A better explanation or a video would help.
In the meantime, from what I understood of what you said you can try this
Cannot join servers with mods (game window never starting)
Stay in the main menu or close your game if you're already stuck without the game window opened.
Open a command prompt (cmd) in Windows and paste this command (right click) rmdir /s /q %localappdata%\Plutonium\storage\iw5\mods then press Enter
Until Plutonium fixes that issue you'll have to do that when the issue occurs.