Lua hacks screwed up
Since, Garrys mod switched to 'steampipe' the more shitty steam, I moved all my files /lua to common/gmod/gmod/lua now when i run lua files, all it does is disable all the gmod commands like quit,disconnect,esc and stuff like that.
Alt F4
On a serious note, did you overwrite any files? Try a game cache validation and/or defrag, and if that does not work a reinstall should most likely fix it. I had the same problem, transferred my files over without overwriting anything, validated files, Voila.
Ik about alt f4 anyway i got this fixed.