Gray screen

I open up One for All, ban, pick, and all, then... Enemy champions show up, friendly champions show up, and the client goes gray screen. For the next fifteen minutes, I try everything I can to get my game back. And when it finally responds, I'm about to go help my team when I realise that it only got me back in because the game ended. It said when it first gray screened that there was an error with connection. There wasn't. What happened? Someone help pls