Google’s Vanessa Fox explains in Google Groups:
If the host is down when Googlebot tries to access your pages, then those pages may disappear from the index until Googlebot can crawl them again. In webmaster tools, do the pages you want indexed appear in the crawl errors section? If so, then Googlebot was unable to access them.
If you are moving the site to a new host and the pages are available the next time Googlebot tries to access them, then you should see them in the index again soon after that.
Vanessa also adds that if this happens to your site there is no reason to request reinclusion, as Googlebot will keep rechecking for it automatically.