@dae will have to check if there is a backup on ankiweb to restore it. It is always best to create your cards on Desktop anyway, since backups are created regularly.
@dae, I think this is another case of the situation mentioned on other threads like:
Looks like the one-way sync dialog is not coming up in these situations (creating cards on Ankiweb, or in my case when I connected to Ankiweb from desktop for the first time on a new computer). I’m not sure if this is by design or if it’s a bug though.