OSX provides home folder encryption system called FileVault. FileVault is effectively a sparsebundle file in your home folder, storing all your data, and is mounted as a loopback volume during the login.

If your Mac loses power on a wrong moment FileVault might become corrupted. This will lead to a situation that you cannot login to the system anymore. The login dialog will just tell “Volume is corrupted. Would you like to repair?” And then it says something along lines “login failed”.

Don’t lose your hope here. You can usually still fix the corrupted FileVault sparsebundle file as root user.

1. To create a root user

• Use OSX installation DVD to add new admin user to the system with non-protected folder
• …or in the case you have non-admin user accounts on the system already, use them to login, navigate to System preferences -> Accounts and create a new admin account.  Note that you can use your credentials from the corrupted user to make admin level system changes even if the logged in user is not admin – just click the lock icon in the System preferences.
• An admin user can enable root user. To enable root user, you need to navigate to /Macintosh HD/System/Library/CoreServices open Directory Utility and choose Enable root user from the menu (Snow Leopard)
• Logout. Then login dialog should have new option “Other” where it allows you to type in username and password. Now login as root with the password you set for it earlier.

2. To fix the damaged sparsebundle file

• Login as root
• Open Application -> Utilities -> Disk utility
• From menu choose Open
• Choose /Macintosh HD/Users/corruptedusername/corruptedusername.sparsebundle file
• Disk Utility will complain about something it cannot mount volume, but the volume still will appear in the right hand disk list
• In the disk list choose the corrupted volume
• Hit repair
• The repair process will take minutes, but completes succesfully

After the repair process is complete, logout as the root user and login as the user who had damaged FileVault. This time the login success.

3. More info

• Repairing a FileVault protected Home folder

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