Are you attempting to redownload the purchased music from the iTunes Store, or are you just seeing in the store that the song now has a price attached? It is not unusual to have a song changed where the publisher removes a version and adds a remastered one that is different from your previous purchase. You do not get that new remastered version for free, but should still be able to redownload the previous purchase using iTunes.
Redownload music from the iTunes Store on your iPhone or iPad - Apple Support
There are cases where Apple loses the rights to a song and will not allow you to redownload it, but as noted in this Support Article, it is not likely. Apple does recommend to permanently download any music content when possible where Apple cannot remove it from your device, even if they no longer have the same rights to the music content.
About purchasing movies, TV shows, music, books, and ringtones from Apple - Apple Support