Can't be sure, but from what you said it appears you are on a login screen.
Since you're using lion, you must boot in recovery mode. Either restart the computer holding the command & r keys or restart the computer holding the option key and choose the recovery disk.
You have to boot from a Lion Recovery drive, this can either be a recovery disk made with the Lion Disk Maker tool, or just by booting from the built-in Lion Recovery HD partition.
- Hold “Option” at boot and select the “Recovery” disk at the boot menu
- Wait for “Mac OS X Utilities” menu to appear, indicating that you are booted into recovery mode
- Click on the “Utilities” menu and select “Terminal”

- Type the following:
resetpassword
- Confirm the user account and then the password change and reboot Mac OS X 10.7 as usual with your new password
This replaces the “Reset Password” menu item that used to be in place prior to OS X 10.7, which was one of two original methods of resetting a Mac OS X 10.6 or prior password. Why the change to the Terminal method? Probably for increased security now that recovery partitions are standard with Lion.
The above method is by far the easiest, but if it’s unavailable for some reason, you can choose another method: