Hi pgan
You have overwriten the database on ankiweb with your local (read: Anki Desktop) database which was still empty. Anki should have automatically made backups in the past so you might be able to recover your decks.
Open your Anki Desktop version, and go to
- File >> Switch Profile >> Open Backup
Follow the instructions and hopefully an automatic backup can help you restore your decks.
With kind regards
Strix