Dropbox stuck uploading and downloading on Windows

Dropbox has been happily working in the background for several years, keeping a useful backup of my important data, which I store in a folder named "Dropbox".  However, without me noticing it stopped working about two weeks ago, so all the work I have done since then wasn't backed up.  When I realised this I hovered on on the Dropbox icon at the bottom-right and it says "Indexing 40,023 files, Uploading 19,948 files, Downloading 23,138 files".  I left it for several hours but nothing changed.  Resource Monitor shows the Dropbox.exe is using 18% of CPU resourced and constanlt reading and writing file on the hard drive.

I read that a possible solution is to open Dropbox >> Preferences >> Accounts, press Alt anywhere in the window then Fix Hardlinks and Fix permissions, but that makes no difference.  I also tried re-booting the machine but that made no difference either.

In desperation I uninstalled Dropbox then re-installed it.  However, that was disasterous! it downloaded all my data from the cloud (which was several weeks out-of-date) and loaded it onto my laptop with today's date! Fotunately in many cases it retained the original folder, renamed named "xxx - copy" and created a new folder "xxx" with the downloaded data.  I had to look through all my Dropbox folder, deleting each "xxx" folder and renaming "xxx - copy" to "xxx".

Fortunately I also do a local backup every night of most of the folders in my Dropbox folder so I was able to restore some of my data from that.

However, even after that the original problem still persists so Drobox isn't backing up.  A further issue is that although I have set Dropbox preferences to Open Dropbox upon startup, it doesn't open at startup.  Furthermore when I checked Startup Apps in Windows Control Panel it shows Dropbox as disabled.  I re-enambled it, but next time I re-boot it is disabled again.

What's gone wrong with Dropbox!  How do I get it working again?