I had a similar problem. I downloaded free recovery software that allowed me to scan the unwritten parts of the hard drive bit by bit to find the lost parts. It was work, a day or so, but it was a lot larger than 2 days work, it was the whole thing. I don't recall what it's called. But don't write other things to the hard drive in the meantime. Google to search and find similar software. The lost backup is there on your hard drive.

Most people today would say, backup automatically to the cloud each night, and save to a USB stick, they're cheap enough.

but it wasn't there either.

then it somehow didn't get saved in the first place, for some reason the program didn't register it.

I finished a novel recently (my first and last). I started it in Pages. Same thing happened to me, I lost 9 hard-won pages of writing that had been on the screen when I did a Save and then Save As elsewhere. Gone. I left Pages, tried LO as well as two other Word-like apps. Finally paid for Word. Too many tutorials were teaching how to do stuff in Word, which, no longer being the computer whiz I used to be, did me no good.

Anyway, my sympathies for the loss. Put it in two different apps altogether in the future before closing up?

I have all of my LO files backed up hourly on a separate external drive. I'm on a Mac, and the backup software is Carbon Copy Cloner. I have had no problems with lost files, but I have experienced one or two files that got corrupted. LO was able to repair the files.

I'm back in business.