Check your server logs to see if it gives any indication of how it was done.
Contact your host to work out if your server was hacked their end or someone has installed a keyboard logger on your machine at home and just logged into your account with the login/password. Do you use the same password everywhere?
If you want to get the site back up and running quickly then get your password changed with the host company and just change the index to forward to your forum.
Thanks for the help, pardon my ignorance but how do I check the server logs? I can see RAW access logs in cpanel or HTTP access logs?