A Microsoft file hosting and synchronization service.
Same thing happened to me. I thought I was going crazy. I knew I saved several things to my new laptop in early 2023. I created a folder on my OneDrive. After creating all these documents, I didn't need to use them again until this year. I KNEW I saved them. I even save different versions of the documents depending on the name of the company it pertains to. They're all gone. I'm using the "free" OneDrive version with 5GB storage. I didn't even realize that Microsoft was copying my stuff to the cloud and deleting things once I hit my 5GB limit, but it continued to add recent things. I'm a student and part of my things that were deleted were things from my Spring 2023 semester. I am upset. Not sure how this is even legal. The thought never crossed my mind that I'd run out of space on my own laptop that has plenty of storage. I don't care about OneDrive and didn't realize I was even using it when I saved things to Documents. I thought it was saving to my computers hard drive.
This is another post that has replies from Microsoft customer service. This is a known issue. https://answers.microsoft.com/en-us/msoffice/forum/all/missing-onedrive-files-all-folders-and-subfolders/ccedf19c-1898-4502-a324-b4eb78a6aa1a?page=2