After power outage cannot login to OS X Mountain Lion

Try this:

Assuming that you are using a recent build of OS X, go to /Applications/Utilities and launch the app called Keychain Access. Go to the Window pulldown menu and select "Keychain First Aid". Enter your password, set the radio button to "Repair", and click "Start".

And, in case it doesn't help:

Boot to the Recovery HD:

Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.

When the menubar appears select Terminal from the Utilities menu.

Enter resetpassword at the prompt and press RETURN. Follow

instructions in the dialog window that will appear.

Or see Reset a Mac OS X 10.7 Lion Password and

OS X Lion- Apple ID can be used to reset your user account password.

OK. You definitely have a login problem if the field shakes. That means wrong username and/or password.

As I earlier posted:

Boot to the Recovery HD:

Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.

When the menubar appears select Terminal from the Utilities menu.

Enter resetpassword at the prompt and press RETURN. Follow

instructions in the dialog window that will appear.

Or see Reset a Mac OS X 10.7 Lion Password and

OS X Lion- Apple ID can be used to reset your user account password.

At risk of being the "unhelpful" one who has no immediate advice, I will say ...

There is a device called a UPS (uninterruptible power supply) that has batteries inside to run a system if wall power fails. The batteries also run the system when power drops from 120 V to 90 V (can happen for very brief periods) that woud cause un unexpected shutdown. When the OS stops "midtsream" it can require repair, as outlined above.

I like APC UPS. I would buy at least 1300 VA.

I'm having the exact same problem. The field just shakes and I can't reset the keychain because I can't log in. Extremely frustrating. I've reset the password and its been no help. Did the restore from back up work? I'm hesitant because mine isn't very recent so I'll lose some docs/pics.

It's a one simple solution... boot from emergency disk use Utiliti->Terminal go to home directory (cd /Volumes/Your_disk_name/Users/Your_user_name/Library/Keychains delete file, or rename it (You can repair it in next normal login), then use reset password (from terminal type ressetpassword), then set new one, save and reebot!

What screen appears when you boot?

Can you boot into Safe Mode?

http://support.apple.com/kb/ht1455

Does it try to log in but can't, or does it just shake the password field?

Did you repair your keychain as suggested before changing your password?

What backups do you have?