I looked in my library and saw that some of the artwork for my songs was gone. I tried to play them, but they wouldn't play. I've looked at other posts and I can't seem to find a way to fix this. All the songs are MP3 and none of them are read only.
This sounds like file corruption. The files are probably damaged, and can no longer play or read the artwork. If you have Time Machine, right click one of the songs and choose "Show in Finder", then locate an older version of the file in Time Machine and replace the current one.