Purchased music disappearing from folders on my PC

I'm finding that certain music purchased and even recently played keeps disappearing from my PC. The folders are where they should be and have been for years, but the actual music files are gone. Searching my computer does not help. I can find them in the ITunes store, and play them from the Store which shows them as purchased. But I cannot download them.

My user name has not changed. And some purchases remain without the mysterious issues. What's the problem here?