If all your decks were deleted, do this:
Please try resetting your local changes. To do this:
Go to browse in your Anki → at the top click on ankihub addon tab → press reset all local changes
This will restore everything to what AnkiHub has unless you have specifically protected those fields
This might have happened if you clicked download to ankiweb, did you see a message about upload to ankiweb or download from ankiweb before this happened?
Make sure to click upload to ankiweb