You need to prove that you own the account, by signing a message using an address used on your hacked account.
Read more about this: Recovering hacked accounts or accounts with lost passwords
https://bitcointalk.org/index.php?topic=497545.0
EDIT
You beat me to it by 8 seconds.
This is cool! I didn't know there was that way to recover the accounts. I hope I never lose it, but I'm quite more relaxed to know that it won't be lost forever if I can sign a transaction with an address that I've proven to be the owner of.
If he/she no longer has access to the account, then they can not sign the address linked to the account. It could only have been done when they were actively in control of the account. Which if they did he would have to follow the instruction in the link already provided on the first comment.
assistance there.
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Gloverwrt, I understand that they're talking about signing with a Bitcoin address, it's obvious that if you lost access to your account you won't be able to do anything from it. I think it's that way because I'm not used to "signing" anything with an "email address", but now you created the doubt on me