Hi, I have just done the same thing where I synchronised my Ankiweb and it has deleted all of my Ankiweb cards. Is there any way that i can retrieve these so my anki web goes back to normal and I get all of my original cards back?
Was the collection on AnkiWeb the most up-to-date version of your collection – i.e. it had changes (edits, review history, etc.) that weren’t synced to any of your devices?