The password doesn't auto change.  Either you have made an error in entering it such as using upper case instead of lower case or have changed it and forgotten it.  In that case use the I forgot my password e-mailer to reset it.

Do you have any other staff members with admin status?  Ask them if they have changed your password.  This has happened in the past.

Also when asking for help please post a correct forum URL so we may help you properly.