If you host goes down it takes ur site down as well. If googlebot happens to visit your site, It will not be able to crawl your site including not downloading robots.txt and sitemap.xml if you have them in google webmaster. This would obviously get you down the ranking. It also depends on how authoritative your site is. Less authority means greater chance of loosing traffic.
Is it working fine now ?? It is suggested to have a page Like "Under Construction" or something like that if possible ... if the chances are that they wont be able to make ur website running within next 24-48 hours better change your host right now !!
they can temporarily out your site from search, but will be back as they find your site is OK. get your site up now. and wait for spider.
My site was once down for 2 days, I then lost all my ranking. Its been 3 months and I havnt got my site ranking back