TL;DR: OneDrive stuck on “Processing changes” for hours

The fix only. No explanation, no diagnosis. Read the full article if any of this does not match what you are seeing.

1. Give it time first if you just copied, unzipped or moved a lot of files. "Processing changes" is normal while a big batch is worked through.
2. Click the OneDrive cloud icon and look for files marked as needing attention. Fix or rename anything with a name Microsoft does not allow (see the list in the article).
3. Pause and resume: cloud icon, More, Pause syncing (2 hours), wait a few seconds, then More, Resume syncing.
4. Check the basics: OneDrive storage is not full, the drive has free space, OneDrive is up to date, and no single synced folder holds more than about 300,000 items.
5. Still stuck? Reset the sync app. This does not delete your files. Press Windows key + R and run:

%localappdata%\Microsoft\OneDrive\onedrive.exe /reset

If Windows says it cannot find the file, run one of these instead:

C:\Program Files\Microsoft OneDrive\onedrive.exe /reset
C:\Program Files (x86)\Microsoft OneDrive\onedrive.exe /reset

6. Start OneDrive again from the Start menu (type OneDrive). It will check every file again, which takes a while.

After a reset you have to choose your "sync only these folders" selection again, for each account.

7. Last resort: cloud icon, Settings, Account, Unlink this PC, then sign in again and point it at the existing OneDrive folder.

Do not move files in and out of the OneDrive folder while it is processing. That creates duplicates and conflict copies.

← Full article, with the diagnosis and the why