GarrysMod keeps on crashing on startup!

GarrysMod keeps crashing and ive been trying to fix it on and off for a couple month now please help! Contact me on Skype if you wish to help (will most likely get spammers but who cares) Skype : lostassassin827 Thanks.

Give me specs of your computer,os…

os - Windows 8
video - AMD Radeon HD 7700 Series
And 8 GB of RAM, so forth but I use to play this game all the time, im thinking that somethings interfering with it somewhere else in my computer.

Try -dxlevel81 in Steam Launch Options

already did

Garry’s mod works for you later? Did you played it later on your computer?

yes I use to play it all the time but one day I jumped on and it just started crashing

How long you have installed windows?

Have you installed a new addon lately? If so how big was addon?