GMod keeps crashing

Every time I exit out of GMod one time, the thing crashes and I can’t get back on it or play any other Steam game unless I restart my computer.

Steam keeps saying it’s running, even though I had already exited out of the game, and I can’t exit out of Steam because of it either. The only way to close it is to restart my computer. It only happens RIGHT after I exit out of the program.

It keeps telling me:
“Steam cannot open this program until the other programs are exited out of.”

What’s going on with this exactly?