Where did my songs go?

I made up a new playlist to burn for my Mother a couple days ago. When I hit the “burn” button, I got a pop-up telling me that several of the songs I had selected were “not found” and did I want to search for them. I searched and finally gave up. Several of my songs from my itune library have exclamation points after them, both on the playlist and in the library.
I’m wondering if I was only able to burn them once or twice, or if there has been some retroactive copyright issues.
The site is baffling to me and I was unable to navigate it enough to get an answer. Can anyone enlighten me?