Can't Launch My GMod!!!!?

I can’t launch garry’s mod and I have not even the slightest idea of where to start. Can anyone help? ANYONE???

We need more information. What happens when you try? Do you get an error message? Does nothing happen at all? Also, please post your computer’s specs.

No error message, but my screen goes black for a second, like when i usually launch, then it flashes my desktop, then black again, then the game disappears and i go back to my desktop.

And computer specs?

try -dxlevel 80