Get session of DNN in aspx file

The aspx file executes outside of DNN. There is no context for you to work with. A much better approach is to turn your .aspx into a .ascx and derive from PortalModuleBase as the start of a DNN module. You can add a proper module to the page and permission the module as you see fit and DNN will take care of all the authentication and authorization details for you.

A starting point for module development

I don't recommend it in this situation, but you can also emulate the approach I described for getting context in a web service.

http://stackoverflow.com/a/9164573/51851