Try resetting your PRAM and NVRAM: http://support.apple.com/kb/ht1379
If that doesn't work, try resetting your password through single user mode:
- Start your computer in single user mode: http://support.apple.com/kb/ht1492
- The computer will show a black screen with white writing. Do not freak out. This is supposed to happen.
- Type the following lines, and press enter after each line:
/sbin/fsck -fy
/sbin/mount -uw /
Important: Wait until you see the "root#:" prompt before typing a new line.
4. Choose the correct line below for your Operating System. Type the respective command and hit enter when you are done.
For Snow Leopard and Below:
launchctl load /System/Library/LaunchDaemons/com.apple.DirectoryServices.plist
For Lion and Above:
launchctl load /System/Library/LaunchDaemons/com.apple.opendirectoryd.plist
5. Now, type ls /Users
This will list all of the users of your computer.
6. Using the name of the account password you want to reset, type this:
dscl . -passwd /Users/USERNAME NEWPASSWORD
Make sure to type the username exactly as it appears in step 5. For example, if the username is "bob72", and you want to reset the password to be "123", you would type the above command as:
dscl . -passwd /Users/bob72 123
7. After executing the last command in Lion or newer operating systems, the computer may return an error about com.apple.DirectoryServicesLocal.plist. Ignore this message! If you properly typed the commands, the password reset worked anyway.
8. Once root# reappears, type reboot then hit enter. The computer will restart, and the account will have the new password.
9. Unfortunately, this process will cause problems with your keychain unless you suddenly remember your old password. When the keychain error appears, hit RESET KEYCHAIN. You will lose any saved passwords. While this is annoying, it is a necessary security feature.
I hope this helps!