Can't load any Minecraft world

I've been trying to play Minecraft for the past two days, however it won't let me. When I try to load up a world it gets stuck on the loading screen, and I've tried all the ways I can think of to play. I should mention that I'm in beta, but if it were a widespread problem I think everyone would be reporting this, and they would've fixed it. However something that is interesting is that I can play if I make a new world. Just wanted to know if anyone else is having this problem.