A Microsoft file hosting and synchronization service.
I have the exact same problem. Using One Drive for years on Laptop running Windows 10 and i-phone with the latest ios updates. All my One Drive files can be accessed from my i-phone but the personal vault is now empty. On the Laptop, everything is there. I tried all the fixes listed on Tommy's response and the vault is still empty. This is very concerning because I rely on mobile access to these important files.
What next?