Hm. Yes, more information would be helpful, but maybe it's one of these things (directly from the BIG help thread):
List of things to check:
1. Wrong username or Password?
2. Do you have the most recent patch?
3. Check so there's no security software blocking the game.
4. Make sure that you've a solid internet connection.
If it's one of those, make sure to check out
this thread. It has more information and explanation on how to fix these problems. C: