Incorrect password after auto-update on iMac

My daughter was using my iMac last night and logged out when she was finished. When I tried to log back in, it said I had the wrong password. I tried changing the password and it worked, but I still get the message that the password is wrong when trying to log in. My computer only has one user account. I have it set up to auto-update software, so I assume it's running the most up-to-date OS, but I can't say for sure at this point.

[Re-Titled by Moderator]