blackscreen during championselect

when i enter a champion select, blackscreen apears and i cant select a champion. i cant see champion select but i can see my friend list i can chat whit my friends during champion select i can see my chatrooms and things like that just champion select is black. i bearly found a video on youtube wich shows exectly what my problem is and i contacted a author of a video and he told me that he had this problem just once and it worked every time after that once in video. i tried many times again and same thing hapens again and again.this problem have not started after pach it juts started suddenly i reinstalled whole game and it didnt worked.please contact me if you have a solution of this problem.(sory for my bad english xD)