Unfortunately you gave very little information in your post and omitted to say how long you've actually had the laptop for since you received it. If it's been longer than thirty days since the date of delivery you'll be out of time to open an eBay case. If the thirty day cut-off date hasn't passed yet then follow the advice given by 61StephenUK.
In the event that it has been more than thirty days since you received the laptop you've got two other options as to how to get your money back. Given that eBay won't be interested in helping you if you've left it for more than thirty days log into your PayPal account and open a case against the seller there. Make sure that you escalate the case before PayPal's twenty day cut-off point, otherwise the case will time out automatically. Unlike eBay, who only offer you thirty days to open a case, PayPal offer buyers a total of one hundred and eighty days to open a case against a seller, so if you paid via PayPal then you've got plenty of time to open a PayPal case should you need to do so.
If you paid for the laptop via a credit or debit card, then if you've run out of time to open an eBay case PayPal won't be an option as they were never involved in the payment side of things. In this scenario you should look to do a chargeback in order to get your money back, so long as the date that you made payment for the laptop is less than one hundred and twenty days ago.
