Does your site serve any
HTML pages? Maybe you have a typo in the .htaccess file and your (shared) host suppresses the error. You could delete/rename the .htaccess and see if that restores proper functioning. Apache doesn't have very good error messages for .htaccess problems (usually it displays an error 500 and exits). You could also ask your provider for HTTP/PHP logs related to your site, assuming they don't provide direct access.
If you're looking for someone to hire, http://www.typo-it.com/
is a phpBB-SEO moderator and does this sort of work.