Are you under ANY obligation to hold onto the data? What was the deal with the backups. Had you taken it when they moved to a new host, did you get paid for it, etc.
I would say you are only under obligation to retain the data for a short period of time, which is after the job was completed.
I would possibly re-send an email and give them a 2 weeks notice tops, but again you are under no obligation to do so.