Can't login with existing admin password

I bought mac book air yesterday. I setup a amin account with password (apple id can be used to reset password). I remember the password very well. I left the mac book running, it upgraded the os itself, restarted then I left for day. When I returned back it is locked and I can't login using my existing password (for admin account). Is this bug with OS?

How do I reset password now? I can login using guest user (which doesn't require password) however under users group to unlock it asks for admin password which is not working. What has happened to my new machine?