Thumbnails Disappearing

Hello all,

I'm new to this forum and thank you for any assistance you can provide.
We're dealing with a number of problems with media pro. The most recent problem is the disappearance of thumbnails. I did not see much on the forum about this issue and wonder if anyone could give me ideas on how to restore lost thumbnails aside from just restoring the catalog from backup. I'm trying to understand why they disappear in the first place.

Thanks!

Al