User Missing From Login Screen

My brother called me up this evening stating that his user was missing from the login screen after the computer froze and was hard rebooted. The issue was resolved by replacing his user .plist file with a copy taken from the most recent Time Machine backup.

Symptoms:

1. The game "Spiral Knights" was updated
2. The computer froze
3. The computer was hard rebooted
4. The login screen had no users. Only the username and password fields were present.
5. When the correct username and password were entered, the screen shook as if incorrect credentials were entered.

Steps Taken to Remedy the Issue:

• Boot up in Recovery Mode
• Launch Terminal
• Check the user's .plist file

ls -alh /Volumes/Macintosh\ HD/var/db/dslocal/nodes/Default/users/<username>.plist

The file size was zero

-rw-------  1 root  wheel   0 Jul  1 20:42 /Volumes/Macintosh HD/var/db/dslocal/nodes/Default/users/<username>.plist

Locate the user .plist file in a Time Machine backup

ls -alh /Volumes/<DriveName>/Backup.backupdb/<ComputerName>/<MostRecentDate>/Macintosh HD/var/db/dslocal/nodes/Default/users/<username>.plist

Copy the .plist file from the Time Machine backup to it's proper location

cp -f /Volumes/<DriveName>/Backup.backupdb/<ComputerName>/<MostRecentDate>/Macintosh HD/var/db/dslocal/nodes/Default/users/<username>.plist /Volumes/Macintosh HD/var/db/dslocal/nodes/Default/users/<username>.plist

Validate that the file size is no longer zero (The file size is different for every user)

ls -alh /Volumes/Macintosh\ HD/var/db/dslocal/nodes/Default/users/<username>.plist

Reboot the computer

That's it folks! I don't have any idea what actually caused the .plist file to blank out, but my suspicion is the computer locked up while editing the user preferences or settings in the file causing it to blank out. Fortunately we were able to resolve the issue without any data loss, much to the surprise and delight of my brother.