plz help me,it's very imprtant for me

Question

i have default page in which i put login contol
when i click on login button i want master page
but i gives me error like
Description: The type of page you have requested is not served because it has been explicitly forbidden.  The extension '.master' may be incorrect.   Please review the URL below and make sure that it is spelled correctly.

Requested URL: /practice_project/MasterPage.master