GMod won't start

I was watchin some vids so i put garry’s mod in windowed mode. I clicked the X button by accident in stead of clicking quit game. Ever since my gmod will not start at all. I even re-installed it but noting works. Please help!!! :confused:

Open the “Launch Options” tab under the settings menu for Gmod. Put in the text box this exactly:

-fullscreen

press ok, and see if it’ll start up.

thank you so much