The tab id for your portal in your site's database got set to the home page tab id. There forth, when the login link is selected the home page is displayed per the tab id. The link below is an article that better explains what caused this condition and how you can fix it by remotely connecting to your database.
http://www.mitchelsellers.com/blogs/tabid/54/articletype/articleview/articleid/124/default.aspx