Black screen bug

Help me please !! file:///C:/Users/Bobi/Desktop/LolClient%202014-10-29%2017-50-53-50.bmp

We can't see that file, just so you know.

But a good step to start in is always trying to repair the client, when you open the League of Legends Launcher, before you click the Launch button, there is a little gear at the top right corner, click that and then click the repair button, it takes a while, but it's always a good place to start with problems.