Those who just had people start using it are lucky. I had an account I had canceled last July, then I get an account ban notice beginning of December due to 'illegal game market activity'. Didn't really care since I wasn't using it but wanted to straighten things out in the event I ever wanted to come back for a while for a new expansion.
It took Blizzard 3 weeks to clear the account of suspicion, failed to actually unban it until a week and 4 email conversations later, and when I finally got in (they gave me a week of subscription to sort stuff out - remember this was supposed to be a hibernating account) I see the hackers had sold everything off 3 of my characters (2 70s and an 80) and drained all funds. Blizzard basically gives a blanket 'here's 2k gold and we'll reinstate about 6 items on the 80, take it or leave it'. If I had really been playing I'd be pissed that I was locked out for a month and that instead of just rewinding the character to when I last played they give you some one size fits all inconvenience reimbursement. The gold wouldn't even cover what was in one of my bags, let alone everything.
Account is sleeping again now, but based on the experience I'd be hesitant to even use it again when Cataclysm arrives