Game freezing.

So I've had this happen twice in the past week. I'll be playing a game, and randomly it'll just freeze. All my stuff will still be running in the background just fine. Browser will still be playing music. But everything above my taskbar is blacked out. Crtl Alt Delete, bring up task manager, everything is still blacked out. I can't close league. Can't get task manager to close it either but the thumbnail tells me league isn't responding. Have to restart my comp to fix this. What's the deal?