Game will only run in Windowed mode [CLIENT]

Pretty much as the title says. Whenever I try to join a game I get through Champ select without any issue, but when it gets to the loading screen it just goes completely black, except for the cursor. The cursor still changes to the hand and I can move it around. I know I join the game as I can still type and I can hear all the sound effects. I was playing with a mate when this first happened and he confirmed that I had joined.

If, however, I switch to windowed mode (either by pressing Alt + Enter or by going into settings) the video comes back. If I play in windowed mode and then switch back to full screen the image freezes. This is a recent bug that randomly appeared a few days ago. I have played games since patch 7.5 came out so I assume it isn't related to that. I haven't installed any new software or hardware on my PC. I've also tried a full repair but that hasn't fixed it.

Even though it works in windowed mode, the framerate drops to such an extent the game becomes unenjoyable.

I'm happy to post screenshots but they will be either a black screen or a screenshot of the game, so not sure how much help they would be. The steps to recreate the bug are literally just joining a game. It happens 100% of the time. I assume it is a client issue, but may also be a gameplay bug.

Any suggestions?