TOPIC: Please help / Reason for inactivity

I have been inactive because of a little problem, no idea how to fix.
When I am playing Garry's Mod, sudden everything stops, but then it works again, but not everything.
I still hear the sounds when people are talking and footsteps and that, but the screen is all black.
This happens to me every time, and it is really annoying when I am waiting so long to be Murderer and this happens T_T