About a week ago, I was working in a test environment for a new site. I left for an hour, came back, and now cannot get to the admin page, as in ‘http://magento.localhost.co
Before anything check your configuration file ( app/etc/local.xml) and make sure that you have "admin" as value for the frontName tag.
ex.:
Usually when you try http://yoursite.com/admin it gives you the admin area Try using an url like that http://yoursite.com/index.php/admin and if it works probably you need to only modify the rewrite rules or follow the suggestions (see the link below)