You can't just 'show a page' ontop of another. You can pull content via AJAX and display it ontop of an existing page. You can then use something like BootStrap's modal dialog to display it. Use jQuery, or something more involved like angular.
The other alternative is to use an i Frame and wrap that into a modal and display that in a modal dialog. That would allow you to use an exiting page. However, iFrames are very finicky in how they display and it may not actually work inside of a modal. The first approach is most likely what you have to do.
+++ Rick ---
>Hi,
>
>I want to create an ASP.NETpage that would be shown as a pop-up/modal form over the existing page. This ASP.NET pop-up form will be used when user clicks on Forget Password link to recover his/her password. The ASP.NET pop up form will show text, email address textbox, and Submit button. I have seen sites where such a form is shown over the existing page and I would like to duplicate this look.
>
>So, say I create such a small (relatively) ASP.NET page; how do I show it over the existing page?
>
>TIA