@GemmaG, I wonder if this isn’t related to Google’s push to prioritize SSL over non-SSL. Perhaps they started checking both HTTPS url’s along with HTTP url’s around the time you saw your rankings drop.
When I go to your website via HTTP, your site loads. If I change HTTP to HTTPS, I get a certificate warning (varies depending on the browser). If I then tell it to ignore the warning and proceed to the site, I end up at a HostMonster login page. That page has nothing to do with your website and has none of your content on it. If GoogleBot ends up giving HTTPS priority and indexes that page and not your non-SSL page, then that may explain your drop in rankings.
It’s also possible that HostMonster made a change to the SSL setup on their servers.
Either way, having someone end up at the HTTPS version of your site and not having your content there is not good!
Get an SSL certificate and get it installed ASAP. For all the trouble this is causing you, the cost for your own SSL cert is minimal.
Make sure that your website content actually loads via HTTPS.
Setup a redirect to send all HTTP traffic to the HTTPS url.