Perhaps your system is booting from an OS on the external hard drive and you have changed your password since that version of the OS.
Did you try to login using old passwords?
You did not indicate if you are unable to login (using your current password) when booting up with the external hard drive disconnected.
I have one of the last white unibody MacBooks (2010). It is updated to OSX 10.9, all other applications are also uptodate. This morning, I had connected an external hard disk to run a backup. When I got back after an hour, the computer had shot down by itself. I restarted it without disconnecting the ext. hard disk. At the password prompt, I entered my usual password. Response: wrong password. I retried 3 times. Same response. It IS the correct password, I use it many times every day. I then signed on using the recovery key (fire vault) password, which worked.
Signing out again to test, if all was OK, I encountered exactly the same problem. It is only possible to activate computer through recovery key. Same goes, when I sign on with my "Admin" account, the (correct!!) passwords are not accepted! Can anyone please please out there help me??
turned out to be a combination of booting from the external hard disc and also something to do with the Mac Filevault. Things only got back to normal, when I had disconnected it (which took a couple of hours to undo...
and also of course disconnected the ext. disc.
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