There are two immediately-obvious possibilities:
1) Your host changed a setting or upgraded something across all of the servers your WP sites are hosted on.
2) All of your sites got hacked in the same way, due to the 'class' of hosting or configuration they all have in common, or because the hacker was able to tell that you had multiple sites, all set up in the same way, and all with the same security vulnerability.
Because of the lack of information as to what precisely is wrong here, I suspect that your best bet is to follow the advice in the error message:
you should probably contact your host. If you still need help you can always visit the WordPress Support Forums.
Tell your host that your .htaccess file is the bog-standard "factory code" for WP, Zend, and PHP, that you don't want to hear the hosting-company-standard-answer "It's your .htaccess file, and sorry, but we don't support .htaccess files," and that you want some concrete solutions or guidance. After spending the requisite 20 or 30 minutes going through their 'standard phone help scripts' again, ask for your problem to be escalated to Level 2 Tech Support, based on the number of sites that you have hosted there, and the fact that they are all down.
In the meantime, start researching responsive and competent hosting, with less emphasis on cheap and easy, in preparation for moving your sites; You get what you pay for, and inexpensive hosting is often the most expensive hosting you can buy -- a fact that I hope is *not* about to be proven to you... :(
Visiting the WP forums, you may find that you're not alone, if indeed this problem was caused by a server upgrade affecting WP installs at GD.
Best,
Jim
