Two days of edits inexplicably disappeared

I have a 2021 MacBook Pro and two days ago I updated my OS to Tahoe 26.4.1. The update seemed to work fine, and I continued working on a Word document I had been editing for the next two days. I always save after every line I write, and I use the auto-backup feature. There was no indication that my saves were not going through. Just now, however, I opened up the document to discover that all my edits (as well as the "last saved" time) had reset to what they were two days ago, around the time of the OS update. This is the case both in Finder and when I go to the Dropbox website, which I use to automatically upload everything I work on to the cloud. This is very alarming. Why has this happened, and is there a way to recover the lost edits? When I try to do "File -> Browse Version History" the menu item is dimmed and I can't click on it. Help!