Okay, first of all, please tell that you did have a backup before pointing Joplin to an empty folder as your sync target, right?
Secondly, if I'm getting this right then your data supposed to be still on your Dropbox account. If you have other desktop device (or another user on current PC) you can try setting up the sync target to Dropbox and sync it. Does it download all your notes?
Thirdly, do you have any other synced device that still has 100% of your notes? If you do, please disable the sync and make an export of all the notes.
Lastly, if all previous steps failed, you can visit this post to see if you can retrieve your notes from Joplin database
Also, a few relevant links for the occasion:
https://joplinapp.org/faq/#all-my-notes-got-deleted-after-changing-the-webdav-url
https://joplinapp.org/faq/#i-deleted-some-notes-by-accident-and-don-t-have-a-backup
