HELP - Our website disappeared suddenly

[Kaycee123]

Our website was working fine until this afternoon and nothing was changed in any of the settings today. All of a sudden we are getting this error message:

The website cannot display the page

HTTP 500
Most likely causes:
<LI id=causeSiteMaintenance>The website is under maintenance.
The website has a programming error.

These are the things I've tried:

I tried disabling all the plugins using the addition to the config.php file of:

define('DISABLE_HOOKS', true);

It still isn't working. I tried overwriting all my files with the original vBulletin files and it still isn't working.

I uploaded a small index.html file and it works fine!

I then deleted everything off the server and uploaded all fresh vBulletin files (after configuring the config file) and it won't let me find the install.php file.

I reuploaded all of the files that were on the server when this problem happened and still nothing is working.

I tried renaming the index.php file to index.html and it comes up okay, but all in script, not the actual index page.

Any help would be GREATLY appreciated!!!