Unfortunately, if someone has hacked into your account and changed your email address, the account will not be secure again. People can retrieve username/password information through any email that has been validated on LiveJournal. You will need to go to http://www.livejournal.com/invite and use an invite code, should you still have one available, to create a new account.
You also might want to use the downloading tool and download any entries you wish to keep so that you may copy/paste them into your new journal. The downloading tool is located at: http://www.livejournal.com/support/faqbrowse.bml?faqid=8 .
Once you have created the account, you will need to make sure it's secure by using the information located at http://www.livejournal.com/support/faqbrowse.bml?faqid=71. You might also pick a different email address to use for that account. Please secure all of your passwords with a mixture of letters and numerals that are insignificant to you (not your birthday) so they will be harder to break into in the future. Also consider using a mixture of upper and lowercase letters as passwords are generally case sensitive.
If you'd like your account suspended to prevent further problems email firstname.lastname@example.org from the email address listed with the account or open a support request in the abuse category while logged into the account.