When and from where?
You have up to 45 days to open an Item Not Recieved dispute in the Resolution Center which is in the top right corner under the Customer Support dropdown menu.
The first step they ask is that you contact the seller. If you have done this, move to the Dispute.
If you have not, don't waste too much time on it. Use the Contact Seller link on the listing to ask for the date of shipping, service used, and Confirmation of Delivery number.
If the item was coming from Canada or the USA allow 20 days before taking that first step.
If the item is coming from Europe, allow 15 days.
If the item is coming from Asia , allow 30 days.
Do not allow more than 42 days to pass before opening the Dispute.
If the seller cannot prove delivery, Paypal will refund you and go after him for the money.