Contact the registrar. If they cannot get the details of the registrant then they will be the ones most likely to advise you further.
If its been recently paid for the the registrar has bank or credit details in the registrant at the very least.
If its been recently paid for the the registrar has bank or credit details in the registrant at the very least.
I think they will probably inquire about the postal address, so I would try mailing a letter as well.
Also keep in mind that not all domains are used for the World Wide Web. I regularly get inquiries about domains I own because "they aren't being used" when, actually, they are critical to my infrastructure and just don't have any web sites associated with them.