A Microsoft file hosting and synchronization service.
Just to echo everyone else, I just booted a laptop that's not been used for a few months and have had the same problem.
Rather than download and update the documents on the unused laptop with the latest versions, Onedrive has uploaded the old versions to Onedrive and replaced the latest versions. It has then deleted all trace of any newer versions (at least from a users point of view) leaving nothing in recycling bins and no trace in 'version history'.
How hard can it be for Onedrive to simply go by dates? I can't believe this could have happened in the first place, let alone the fact that it's clearly a known issue and nothing has been done about it.
In the case of the document I had been working on, I was luck enough to find an up to date copy that LibraOffice had hidden on the laptop I had originally been working on but I'm yet to look to see what else may have been lost. I see little option now other than to stop using Onedrive altogether.