The desktop app on my Mac has been stuck indexing files. I can only access some of them.

I RECEIVED A SOLUTION FROM DROBOX THAT WORKED!

To ensure that Dropbox is working correctly, I'd recommend that you rebuild your configuration settings by unlinking and relinking your Dropbox account to your computer. This will restore any settings which may have been changed without affecting or removing your files.
 
Please follow these steps to unlink and relink your computer:
 
1) Right-click the Dropbox icon in the system tray, or menu bar, by the clock.
2) Click on your profile picture or initials to the right of the search bar, and select "Preferences".
3) Select the Account tab.
4) Click the "Sign Out" button in the Account tab.
5) Your Dropbox will then prompt you to re-enter your account information.
 
Your account will then take a few moments to re-index your Dropbox folder, and sync any pending changes before returning to normal activity.
 

I did this.  On the first attempt, my 29,000 files were re-synched but stopped at 352 files.  So, I repeated the process.  I signed out then signed back in and ALL THE FILES SYNCHED.  I am back to where I was.

In retrospect, it seems a logical maneuver.  This is what we do when our laptop or program locks up.  Just sign out, give it a moment, then log back in.  Good to know for the next update.

GOOD LUCK!