Go here to file a PayPal dispute
cms.paypal.com/cgi-bin/?cmd=_render-content&content_ID=security/buyer_protection_resolution
You have 45 days to file a dispute. Follow the instructions on the PayPal Buyer Protection pages. Do not let the seller talk you into waiting longer than 45 days, even if they swear they shipped it. You can always cancel a dispute if it does arrive, but you are out of luck entirely on day 46.
If it was a service (not goods) that you purchased you may not be eligible for purchase protection, because PayPal has no proof whether the service was delivered or not. cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=ua/UserAgreement_full#13.%20PayPal%20Purchase%20Protection.
Paypal is ONLY secure for goods that can be packed in a box and delivered with a courier waybill and a signature on delivery. Do not use PayPal for face to face transactions or for delivery other than by signed courier receipt.