Mojang not responding message

When I try to play, the game stops loading and  I get a message that reads “mojang not responding” and stops loading. I have to quit the game. I tried to delete the game and reload it but that did not help. I tried previous versions of the game. I was able to get into the game with 1.0 but as soon as I leave and come back the game freezes up and I can no longer play in that world.