You can log into recovery mode from the boot menu in grub. Then you can
use the command line to set it. I would use the following Google searches
to look for how to do all of that:

"how to start recovery mode in ubuntu" (include your version number, like
12.04, or whatever it is.

"how to change a users password linux command line"

A good idea to have everything ppetant on your computer backed up first.

Hope that information is of some help.