Something's messed up with friendly URLs. To get your forums back online turn this feature off (Configuration → Server and Optimization Options); have you renamed htaccess.txt file to .htaccess after enabling it? Either that or your hosting does not support mod_rewrite, required for friendly URLs.
You're using 000webhost, this could be happening for a whole range of reasons.
1. They've suspended you for some stupid reason (000webhost are notorious for this).
2. You've pointed your domain to the wrong DNS servers.
3. You've maxed your monthly bandwidth / or the servers CPU.
This is an issue with the host, and not your forum.
Thanks guys. I hated that thing and it's finally fixed