I had this problem once when my webhost who was troubleshooting another site of mine accidentally deleted my .htaccess file in my XenForo installation's root directory.
That made the site do the exact same thing that yours is now.
I luckily had a backup of the htaccess file, but I'd assume you could turn
full friendly urls (Admin CP > Options > Basic Board Information > Use Full Friendly URLs
) off and then try accessing a forum to see if that is what's wrong.
Code:
# Mod_security can interfere with uploading of content such as attachments. If you
# cannot attach files, remove the "#" from the lines below.
#<IfModule mod_security.c>
# SecFilterEngine Off
# SecFilterScanPOST Off
#</IfModule>
ErrorDocument 401 default
ErrorDocument 403 default
ErrorDocument 404 default
ErrorDocument 500 default
<IfModule mod_rewrite.c>
RewriteEngine On
# If you are having problems with the rewrite rules, remove the "#" from the
# line that begins "RewriteBase" below. You will also have to change the path
# of the rewrite to reflect the path to your XenForo installation.
#RewriteBase /xenforo
# This line may be needed to enable WebDAV editing with PHP as a CGI.
#RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -l [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^.*$ - [NC,L]
RewriteRule ^(data/|js/|styles/|install/|favicon\.ico|crossdomain\.xml|robots\.txt) - [NC,L]
RewriteRule ^.*$ index.php [NC,L]
</IfModule>
If you don't have a backup you could try using mine also, simply place the above code inside a file called '.htaccess' (Assuming you're using a linux based host) inside your forum's root directory.
Good luck!