Never mind - I figured it out. OneDrive had stopped running for some reason. Once I manually started it again, it synced the files.
Files disappearing in OneDrive
My colleague emailed me two files, which I downloaded and opened in Word. I saved two copies of each to a OneDrive subfolder, made edits on one copy of each, saved, closed the files, then went to email them back to my colleague. But when I went to attach the files, I opened the subfolder and both copies of each file had disappeared! I opened File Explorer and tried accessing them through that, but they weren't showing there either. So I went back into Word and went to open recent files, and they were displaying there fine, listed as being in the same subfolder I'd been looking in (definitely the same one - I have other files in there from last week, and they were all displaying fine in the email attachment dialogue and in File Explorer. In Word, I right-clicked the file name of one of the 'missing' files and went to "Open File Location", and there were my files, listed alongside the others in the subfolder.)
I saved copies of the files to my desktop and emailed these copies to my colleague, but I want to know why they're not appearing in OneDrive!