Previously, were you given an address to mail your payments? If so, make the payment by certified mail/return receipt or delivery confirmation with signature requirement. If it is returned, keep the envelope unopened for your records.
There should also have been an address for correspondence (not payments). If there was an address provided, send a letter noting your problem with the number. Again send it by certified mail/return receipt or priority mail with delivery confirmation. Until you receive further instruction, this is all you can do for now. It is likely someone will take over for that firm shortly.
If your payment was through a court order and the payment is returned, you should resolve this in court. Again, many people appear to be hung out to dry because of firm's apparent closure.
