forum index hacked-redirect

[jojojijijojo1]

Greeting everyone,
I own a vbulleitn 4 forum, and I was hacked, my forum index.php displayed a message of the hacker, then redirect to the hackers website, my question is how they could do that, and how to stop such an attack in the furure? what are the causes? also the exploit was used in the database, because re-uploading all the original files did not work, so i had to restore the database.