login screen won’t accept my password
If you cannot get to the login prompt, or your login window appears but shows no username, it is possible your NetInfo directory has been corrupted.
You can try booting off the OS X installer CD and from within the installer > file menu > reset password you can select the user with a failed login password and reset it.
If this doesn’t work or you do not have the OS X Installer CD, you can try this:
On startup: Command-S Single User Mode
Type each of these commands (bold text), followed by pressing the Enter Key at the end of each line break
mount -uw /
mv local.nidb local.nidb.corrupt