Soemtimes it is possible that you have updated the login page information in dotnetnuke and after that you dont have any way to get back this login page. Ironically, you cannot even login to change the login page. You are completely stuck. Now only one way to get back this login page is directly updating the database entry.
Login to the dotnetnuke database using SQL Server Enterprise Manager. Open the “Portals” table. Select the row for which you want to get back the login page. Locate the column “LoginTabID”. Press ctrl+0 to enter NULL value in that column.
Now type the URL of your website and login to your dotnetnuke portal with a smile!!!