A month of comments disappeared from my site & db

I just discovered that a huge chunk of comments disappeared from my WordPress website, about a month’s worth from mid November through December 24th.

I do have db backups that I hope cover the time period but I have no idea how to use them to get the comments back into the active db. I’m hoping my server admin can help me out with that. I tried exporting just the comments part of the db using phpMyAdmin, thinking I could do the same with the backups and then somehow cobble them together again but I wasn’t able to export the comments section.

Any idea how/why a chunk of the db can just vanish? And how to prevent it in the future?

Many thanks!