When the login banner appears, hit Ctrl+Alt+F1 to get a console. Login in as root and use
# passwd username
to change the user account's password. If you've forgotten your root password (meaning you're probably really desperate), at the grub splash screen, instead of hitting [enter] to boot your kernel, hit e to edit the boot options. Then select the line that begins with kernel and hit e again. Add the word single to the end of the line. Hit [enter] followed by b. The system will now boot into singleuser mode. Run passwd to change your root password and passwd username to change your user password.