Yesterday everything worked, today I try going into a thread and I get a 404 error message saying the forum html can no tbe found. Here is a link to take a look
It's SEO problems. Create BLANK .htaccess file in your main DiscuzX directory and go to Settings -> SEO. Click View the current Rewrite Rules and copy content into your .htaccess file. It should work if your server support apache and mod_rewrite module. If no uncheck all fields in your SEO's settings.
By the way. If your forum is only in english I suggest you to set default language.