Karamazov123 OneDrive is unfortunately not as reliable as it could be, and definitely not enough to trust it as a backup. I don't know exactly how this would have happened, but two possibilities are you're low on cloud storage, and OneDrive deleted older files; and the syncing process itself was interrupted or otherwise didn't go through. Did you ever see your saves in OneDrive after syncing and before they disappeared? If not, I'd go with an interrupted or compromised sync, but there's no way to tell.
The only other location in OneDrive where your saves might be is the Recycle Bin, if you haven't looked there already. But that assumes the data made it to cloud storage in the first place.