Characters get deleted after 3 months of inactivity, even if you are still be billed for the contentID(s). However, you can get SquareEnix to restore an old account from backup. Their backups only go back one year, and they will only do this ONCE per person. If you have mules with the account, you have to tell them the name of all associated characters on the account. If you restore one character then later remember the mules/alts, they won't do it, as you will have already used up your one chance.
I don't think they can do anything if it's been more than 1 year, but you can always try calling them anyway. Try visiting the Playonline website for more info.