I would probably try a certified letter to the brokerage office. You might also see if the local or state REALTOR association can help connect you or guide you.
Beyond that, the listing agreement is probably a legally binding agreement, so you should seek legal advice.
They don't have to release you from the agreement in most cases.
it's a contract, but you might read through it and see if you can be released and under what circumstances.
Call, fax, FedEx, go to their office....hopefully one or all of those methods will get you some kind of response from them.
Sorry to hear about this lack of service.