My yahoo mail account has been hacked and I can't regain access to it
because my security question has been changed too. I contacted yahoo
customer service but even they said they can't do anything until I
answer the security question.

My mail account has been setup to forward mails to my gmail account
and I still get forwarded emails sent to my yahoo account. I told this
to yahoo customer service but no avail.

Please let me know how I can force yahoo to give my mail account back.

Thanks

Unfortunaly you can not force them to give it back because it is not
your account it is tecnicly threes. they give you the right to use it.
i am sorry.

good luck