LoL wont load after champion select

After champion select, the loading screen stays black and I get the message: Unable to connect to the server. Please check your network connection and attempt to reconnect to your game. (Options are Retry and Cancel) Also, the logo stays in the middle of the screen if I press alt tab. This has happened 3 times in the past 2 days, but it doesnt happen every game. Does anyone know why this is happening and how I can fix it?