"No Video Signal " after champ select...

I NEED HELP ASAP. I'm not sure if its my PC or the Client. During the "No video Signal" black screen, I can still hear the game in the background. I tried playing other games on steam, everything is normal ( this only happen to LoL ). I uninstall and reinstall LoL three times already and still get the problem. I'll take any advice. ( First time posting ).