When you attempt to sign in if you can get to the first page where you enter your username and password, if you can't seem to log in with your current details click on the "I've forgotten my password" link. That way you should be sent a link to reset your password. Once this happens reset your password to something that would prove to be extremely problematic for a hacker to successfully crack, but easy for you to remember.
After you have reset your password clear all caches and cookies, then once this is done try to log on again using your username (or the e-mail address that the password reset was sent to), then enter the new password into the Password field. If successful you should be able to log back into your account; however, if there looks to have been suspicious activity on your account during the time that you were unable to log in then get in touch with eBay Customer Services and report that you think your account may have been hacked. That way if somebody has accessed your account and changed the password - in which case your own password would not have worked - then eBay will put a temporary hold on your account whilst they investigate the matter.
In the event that your account does turn out to have been hacked delete your card details from the account and report the matter to your card provider. You will then be issued with a replacement card, at which point the old one will be cancelled so that no further purchases can be made with that card.
