Downloaded songs won't play on itunes

I have itunes on my PC, which I use to listening my computer as well as support my ipod. I upgraded to version 12.10.7.3 about a year ago because the version I had could no longer access the itunes store. The new version gave me access to the store again, but most of the songs I downloaded won't play on my desktop, nor sync over to the ipod, including songs I already had which I had downloaded previously (through itunes). When I click on one of those songs, it says my computer is not authorized, so I enter my info to authorize it, but it still will not play, and if I click on it again, it does that again, over and over. I only have 4 authorized devices. I have tried deauthorizing and reauthorizing. I pulled up one of the songs in Windows Explorer and under Properties -> Details, I see that it says it's protected, whereas a download that would actually play is not downloaded, but I don't know how to unprotect it. I know there are more recent updates but I am very hesitant because a) I don't want to upgrade to a version that no longer supports ipods and b) the last upgrade significantly screwed me over and took away access to many of my songs. When I upgraded last time, I signed into my account on my husband's computer, downloaded the latest itunes at that time, downloaded a song to test it would work on my ipod, then signed in on my computer to upgrade. Advice on how to access my music which I paid for and have never been able to listen to would be greatly appreciated!