Disappearing videos in iTunes/TV library

There are a number of videos that I added to my iTunes library over the years, some of which were tagged as music videos, some movies, some tv shows. They were not purchased from the iTunes store. I dragged and dropped them into iTunes and they lived there and I watched them on two different versions of AppleTV (the device, not the app).

I was looking for one of them recently in the TV app, and its title was listed but there was no picture. When I clicked on the play button in the middle of the blank square, I got an error message saying the file could not be found. When I clicked Get Info, I got the same message, but then I could see the info including name, file type, size, last time I watched it, etc. But it still wouldn't open.

So I drilled down into the iTunes library and found that there was a folder with the file's title, but there was nothing in it. And further exploration revealed about 10 -15 other files that had vanished from their folders.

Has anyone else experienced something like this?