Not being able to alt tab in game when I could before

My league was working perfectly fine until about 2 weeks ago when i started to get kicked out of loading screens frequently which lead to bugsplat, from then my league would show a completely black background but i'd be able to see the HUD when in game after trying to alt-tab on the loading screen. I changed the comparability settings and now it doesn't kick me off anymore but i still can't alt-tab in game. I don't know what the problem is and how to get it back to how it was before? Any help would be appreciated