Claudius 0 Newbie Poster

Hi
when I log in fire vault ask me for the password as I type it says is wrong (I swear it is not) anyway after few attempts I have to punch in the master password, I do it and it is fine, it then asks me to put a new password in and verify it, I cannot change the password, I get a message that says unable to log in fo this account. It never happen before, can I get some help?
I have a macbook pro (2weeks old) and Mac tiger