There are a few reasons this can happen - did you get a payment notification to your email address?
Usually, if someone has gone through eBay checkout and paid, the $ symbol next to the item (on eBay) will change depending on the payment status. A filled in blue $ sign means the payment has gone through successfully and the money should show in your account. If it shows an hourglass, it means the payment is pending, and that can be because they paid via eCheque, there was an error in the email address when creating the listing (like a typo, or using an address not on file with PayPal), or because they paid by credit card and you only have a personal account (you need to upgrade to a Premier account to accept payments via credit card, but the transaction should show in your account history, and give you an option to manually accept the payment and upgrade).
If an incorrect email address was used in the listing, your only options are to call PayPal and ask if they can credit the payment to your account, or ask the buyer to cancel the payment and invoice them directly from PayPal (make sure to send an invoice for "goods", and quote the ebay item title / number on the invoice if you do it that way).
