A Microsoft file hosting and synchronization service.
I'm having similar problems with files disappearing. I saved the file I was working on in Word, but when I went to send it to someone it disappeared. I couldn't find it anywhere on my mac, in word, onedrive, the Icloud--gone!
It also made my updates to new files into old files. The August/July files gone!
I started playing around with files that aren't important--a couple of Shakespeare sonnets 1, 2, 3 etc. The same thing happened. I was able to save one or two files, but when I did a rewrite or added something and saved, they disappeared. Bizarre. These are my personal files, but it's happening at work too for lots of people. Some co-workers are having their entire files and data disappear.
I'm thinking of ditching Microsoft and Onedrive for something else, but not sure what.