Make sure you have every scrap of information that has ever been linked to the account: registration codes, credit card numbers, phone numbers, addresses, etc. Since the account was hacked, they'll ask you to list the character names/jobs/levels on the account as well, and you'll be required to fill out a form stating the account in question belongs to you, notarize it, and send it in. They'll also want to know exactly when you were hacked, so they can investigate, and if the account is restorable, it'll count as your one-time-only account rollback, so don't get hacked again or they'll tell you to piss off. (Consider investing in a security token regardless, but especially in this case.)
Assuming the account is recoverable, expect to wait anywhere from 2~6 weeks to get it back, and you can count on losing some of the progress you had made within a few days of when you were hacked.
I suggest doing some research on the current state of the game before deciding to go through with this process; the vast majority of your gear, even if you were geared to the teeth 3 years ago, will be obsolete at this point, the gear you'd be replacing it with is easy to come by for the most part, XP/hour rates have increased twentyfold, and most of the older content you'd have to redo on a new character has been streamlined with the level cap increases / removal of level caps from zones and mission battles. In other words, you could create a new character that surpasses your old one in the time it would take SE to roll back your old account.
In any case, good luck and welcome back!