Start with this:
FTP to the site.
Open includes/
If it is blank or missing info on lines especially the top 7 or so lines then that file got hosed somehow.
Those top lines have all the path and login info for the database and forum.
If it is hosed you can either manually edit the file or upload a file from a backup.

The next is what I mentioned above.
Ftp to libs/
Edit the file search for
$showerror = 0
change it to:
$showerror = 1
Save it and visit the site it will show you what the actual error is.
When you are done change it back to:
$showerror = 0

And enable the logs in the control panel for future.

Blue Man Group
There is no such thing as stupid questions. Just stupid answers