If it is past the estimated delivery date, you can file an item not received case in the Resolution center, below. You have thirty days on Ebay to file and six months at Paypal.
You might want to message the person you bought from asking where your item is. Also be sure to double check if they are currently away. It will usually say in the listing a return date of some sort. You may want to check under the purchase price if they have an abnormally long handling period as it will specify that.