I can't play minecraft multiplayer for some reason, when I try to enter it says Internal Exception:java.lang.NullPointerException

It really sucks I can't play Minecraft multiplayer because I have lots of friends to play with and now I can't play with them. I have had this problem for 2 weeks so I hope you can fix it. I am sorry if it should go away in some time but i have been getting worried and also for some reason my username is my very first old one that is Yahani2007 when i play in single player but i think thats not a big problem.