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This just happened to me in a ranked game... my entire champion selection screen was blacked out. I was team captain so I couldn't ban any characters, nor could I choose my own hero when it was my turn. My teammates said I was showing up as a disconnected character (lightning bolt picture) even though I could chat with them.
First time it happened to me.
I think this had something to do with my choosing masteries. I was in a different game (selecting heroes) and I was choosing my mastery points. Someone dodged that game during champion select (and while I was choosing my masteries) and it automatically put me in a new queue, and it instantaneously found me a new game. When I was placed in the new game is when the problem occurred.