Black screen/boxes appearing during/after game [gameplay] [client]

During a game of ARAM, my screen went black a few minutes in. The only thing that was showing was the cursor, and if I clicked keys I could hear them (esc opening the options window) as well as the bg music/sounds. I had to force quit the game and jump back in then everything was fine. But once the game was over the client looked like the image posted. This has never happened to me before and it doesn't seem to affect my gameplay much, but it is a very weird thing to experience.