You need to be a little more specific. Does it happen when you try to enter a map or as you try opening the game?
It has happened to me before as I try to log in (Or enter a map, can't remember which). That can happen when the servers are really full. If it's that, all I can say is wait a bit and try again.
but if it's the first option I gave at the top, then I cannot help you XD