League not opening.

Soooo, i just got league on my brother's PC. I was able to play when i first opened the game, but the second time i tried it it wasn't working. This happened on the same day. The league icon would pop up but then a black rectangle would appear in the top left corner of my screen. that rectangle was supposed to be my client. i don't know what to try. Some help would be welcomed.

PS: i do not have avast or any other antivirus that could cause such problems. If you need additional info, just ask. Thank you in advance