Go to your Paypal account and open a Dispute/
PP's Resolution Centre is hidden at the top of your account page under Tools.
You have 180 days for this.
If the problem is an Item Not As Described, you will have to prove return to the seller to get a refund. You will not be refunded for your return postage.
If the problem is Item Not Received, the seller will have to prove Delivery (not shipping, delivery) or you will be refunded.