Have you contacted the seller? It`s the first thing to do. Then if he/she doesn't answer after a few days, you open a dispute on paypal. You can find info about this on the paypal website.
You shouldn't be worried. I have bought thousand of items using paypal in the last five years and never got any unsolved problem. Sellers wants to keep a good review to stay in business on ebay. Most of the time you don't even have to open a dispute. But it happened to me twice, and I always got back my money. You are protected for this kind of situation.