Piggybacking on Denis’s comment about the site’s domain having changed….
if you can get access to the database (ie. if your host has phpmyadmin on its control panel) check the ‘siteurl’ option in the wp_options table of the database. If that option is set to an address that redirects or doesn’t resolve, you won’t be able to get into your admin section.
I’ve seen that sort of thing happen before when a hosting provider had a setting that redirected all requests from domain.url to www.domain.url, but the blog’s ‘siteurl’ option was set as http://domain.url