Double check that your password and username are right, and try to log in multiple times. Think back and try to remember if you ever changed your password. If that doesn't work, I don't really know what to tell you.
It could be possible that your account was hacked into and the password was changed, but that is very rare and I don't know why someone would do it in the first place.
Sorry for the bad news if you still can't get in, but I hope you will be successful at logging in. :)