Hi Vick,
This can be handled using the HTTP module.
Create a HTTP Module File
Refer the HTTP Module File in your web.config
so when ever the application loads any page this maodule will be included in the Page life cycle process, so that you can check for the proper user authorization & Authenticatinthrough some session or cookie data if the user pressed logout, probably you may cleared those cookie / session values.
This HTTP Module File, will check for the values if the values are null then automatically redirects the page to Login.aspx or the page specified.
Note: HTTP Module File will be called before Page Load of all pages.
Implementing HTTP Module[
^]
Thanks!