Go to the Resolution Centre and open an Item Not Received Case for the item.
This will expedite your refund from the seller. If you are told you were already refunded, check your funding source for the money.
Don't accept an offer for a replacement unless the seller is located within the same country as you and it will arrive within the week. Offering to send a replacement is often but not always a stall tactic designed to make you wait out your claim.