iPod and Mac: Music Not Syncing

I have purchased and downloaded many songs from iTunes over time, and one day, it didn't let me sync half of my songs (that I fairly purchased on iTunes). All of my songs are still on my Macbook Pro, but I can't sync half of my songs from my Mac to my iPod 4th Gen. I've tried re-syncing, but every time I do, a pop-up shows up and it says, "iTunes Library file cannot be saved. An unknown error occurred(-54)." I know it's not because of storage problems because it still have 8GB left for 123 songs. I also tried authorizing and then manually syncing, and it still didn't work, and I don't know what to do. If anyone knows a solution, please share it with me!