You need to double check that you're entering your login details accurately.
If you are, the next thing I would try is the lost password procedure.
If this also fails I think you will need to contact eBay customer support. It's possible that your old account has been hacked and the password changed to lock you out.
The most common reason for this is that a user has been tricked by a phishing email into giving away their username and password by entering these in a bogus web site, for which a link will conveniently have been provided. Do you perhaps remember anything that might have been a phishing attempt? This would typically be a message designed to panic you by claiming that your account was at risk in some way.
