ipod cannot be synced??

I keep getting this message when I try to sync my iPod:

This iPod "Brittany's iPod" cannot be synced. An unknown error occurred (-54)

I was just tying to add some new songs to my iPod, and I get this every time. It's also taken a lot of my songs off my iPod, for some reason. They show up on my iPod, but they're grey with the downloading symbol (square inside circle) next to them. When I select one, it just shuffles through my songs.

I've tried restarting my computer, checking for updates, checking my firewall, running diagnostics in iTunes, checking for iTunes updates, etc. Nothing is working. Does anyone have any idea on how to fix this?