My friend is having an issue.

For a month or so now, my friend has been unable to play gmod. Here’s his reason.
" I did a very stupid move, by removing the HDMI cord to my LCD t.v. while it was still plugged into my PC to go put it in another room for a bit. As i put it back, the screen was messed up, and it looked as though the PC thought I had two moniters. I restarted, and the screen would just black out after the OS loads. So, I went into my desktop on safe mode, rolled back my GPU drivers, and re-updated them… Everything just needed to be fixed, such as resolution, after, to fit the t.v… but Garrys mod wont work… What it does, is a window of it flickers on then off for a split second. kaput. I used to run Gmod in a window. Perhaps its freaking out? But that can’t be too plausable as the cause, because I ran TF2 in a window too, and its ifne, it just started full screen after the incident."
I tried to persuade him to make an account here and make the thread himself, but no avail. So I decided to make one for him. Anyhow, if there is a known solution to this problem, please, don’t hesitate in posting it.

Perhaps you updated them too far? Sounds strange but some updates aren’t compatible with everything.