Purchased Songs Disappear From Library AND HD

Issue has been happening about five years on two different machines.

I pay Apple for music purchases.

They eventually disappear from my local Library and my internal HD.

I say, heck buy it again. Go to iTunes Store. Says "Purchased" and is grayed out. Can't purchase it again, can't listen to what I purchased the first time, and unless I put it on my ext HD, I can't access it all.