Songs are gone from my iPod but still showing up in Library

I have a 64GB iPod Touch running iOS 5.1.1.

Up until this morning, I had over 4,000 songs in my iTunes Library and on my iPod (mostly uploaded from CDs). I manually manage my Music files. This morning, while the iPod was plugged in to my PC at work, there was a power outage. After that, my iPod would only play files that were bought on iTunes, but nothing else. At the bottom, where it used to show only about 30Gb free, it now shows 55GB.

All the files are still showing on my iPod, but when I click on them, I get the exclamation mark with the circle around it.

I have a backup on my home PC, so I attempted to restore from the most recent backup to no avail. The files show up but they still don't play.

Next, I tried changing the Sync settings from "Manually Manage" to "Automatically Sync". I got some of the songs back but not all. I now have 345 songs on my iPod (I used to have 4,000). All the songs still play in iTunes on my PC but they aren't syncing to my iPod for some reason.

Any suggestions would be greatly appreciated.