You opened a case (item not received?) and the seller isn't responding? If it has been more than 30 days after delivery or 30 days after the latest estimated delivery date it may be too late to ask ebay to step in. If it is after that time they will let you open the initial case/request but may not let you go further than that.
You may have to open the case on Paypal as they have a 180 day claim timeline.