All your content in the Content Page should be wrapped inside
<asp:content ..
tag that corresponds to the
<asp:ContentPlaceHolder ..
of your masterpage.
The simple and quick solution is to create a new page and while make adding the page from Template, make sure that you have checked the Checkbox for referencing MasterPage if you are using Visual Studio to develop your application and in the next screen select the masterpage. Then just copy the content of your existing page which is wrapped inside form tag (normally).
Check this link-
http://www.brainbrushups.com/2013/05/create-master-page-and-content-page-in.html[
^]
Hope, it helps :)
If your problem still exists, please share the relevant code so that we can identify actual issue.