Most are from CDs, some from the iTunes store and others I got from Youtube or just online.And you cannot re-import them into iTunes? Very strange. Did this just start or has it always been this way? If it just started, what is new or different since the last time it worked properly?

Have you tried a reinstall of iTunes (or installing the latest version if it isn't already installed)? You should also POST the problem / question on the Apple Support Forum.This happened maybe 2 WEEKS ago? At first, it was only one song I wanted to play so I didn't care. I just deleted the song. But as I wanted to play other songs, I found they didn't work either. iTunes is up to date.Well, you should post on their forum. And just to be safe, you should PROBABLY run a full scan of your system with BOTH your anti virus utility AND MalwareBytes. Sorry I haven't been able to be of more help.No problem. I'll try their forum then. Doing a scan now.I managed to get it fixed, thanks to the help of the Apple forum. I was told to download "MP3 Validator". It found so MANY problems with my MP3s, fixed them, and now every song works. Just posting again incase anyone else has the same problem and finds this thread. Glad you got it fixed and thanks for the update.