If you can select booting to command prompt some way, possibly by selecting such at the login screen, use passwd command to change user passwd. With Ubuntu it would probably be: sudo passwd user
Replace "user" with your actual user name, after running this command you will be prompted to type a new unix password for that user, enter the password, hit enter and you'll be prompted to retype the password, enter the password, hit enter and it should confirm the changes.
Last edited by Brains; 11-13-2011 at 03:58 PM.
Reason: added for clarification