Black Screen

Sometimes when I'm in queue , It finds a Match but when I press "Ok" to join the match it comes up with a black screen where i cant do anything but close league of legends which ends up giving me a time penalty for leaving the game. I don't know why this happens but the most frequent time it occured when i was looking through the shop and I went into a match. It's really annoying because i have to wait like 15 minutes just to play an aram game. Please if someone knows why this happens , i would like to know. It doesnt happen a lot, only a few times in the past 5 months