If files are missing in your OneDrive folder, or you see the small brown/beige archive box icon overlay on your file icons, and OneDrive is definitely running, and closing/re-opening it doesn’t help, you may need to reset OneDrive.
Note: Check that you’re signed into OneDrive using the appropriate account first.
Quick method: (refresh, 5-10 mins)
Start > Run > wsreset.exe
A blank Command Prompt window may flash by, and the Microsoft Store app may open. Just close them afterward.
Longer method: (full re-sync of all files and rebuilt .DAT file, could take hours)
Start > Run > [one of the following]
%localappdata%\Microsoft\OneDrive\onedrive.exe /reset
C:\Program Files\Microsoft OneDrive\onedrive.exe /reset
C:\Program Files (x86)\Microsoft OneDrive\onedrive.exe /reset