Well, you could
1) Reload the image if you don't care about losing the changes that you made.
Or assuming there isn't a major corruption of the card and you just need to change the password:
2) Read the SD card after starting a Linux live CD, like from Ubuntu for example, and edit the passwd and or shadow files.
3) Or try this trick that I see posted a bunch of times on this forum:
http://www.raspberrypi.org/phpBB3//view ... 47&t=20397
After you are done, so you have another way in, set a password for root. Also, start making backups on a regular basis because a forgotten/corrupted password isn't the only thing that can bite you.