What does tracking say? Contact your seller first, and if they won't help you can file a case in the Resolution Center after the last estimated delivery date has passed, and you have 30 days from that date to open a case. If this is an international shipment it can take extra time to arrive. Do not let the time to file a case expire. Do not close the case until it's resolved.
Resolution Center:
http://resolutioncenter.ebay.com/
This was in the listing. Did you read it?
eBay Money Back Guarantee:
http://pages.ebay.com/ebay-money-back-guarantee/
I am not an eBay employee. I'm a US eBay Community Mentor.
Sunny