Every time I update Mountain Lion and restart my MacBook Air it refuses to recognise my password.
If this happens there is a long way round to get it to open up and a quick, short way. Yes, guess which I tried first:
1. Boot in Recovery system. Press and hold the Command and R keys while you start or restart.
2. When restarted, click Utilities in the top menu bar and select Terminal from the dropdown menu.
3. When Terminal starts, type
and press Return.
4. Then respond to the dialogue boxes to reset your password. You can use the one you had before if you can remember it.
Okay, what's the easy way?
Try to Restart using the restart button in the usual way.
When you get the choice of user identities click on yours and enter your password. If it's refused three times you'll be offered the chance to reset it using your Apple identity. Easy!