Music Video files can't be found on iMac, but play on iPad and iPhone, what causes that?

I have had music videos on my iMac for many years and all of a sudden when I select a video, I get a dialogue box that says "file can't be found", but the same videos play on my iPad and iPhone. What causes that. I spent and hour and a half with Apple Support and all they could determine was that I would need a subscription to Apple Music to play my videos, which I have already paid for. What a rip off! Has anyone else experienced this?