1. Add
<meta http-equiv="refresh" content="60"> </meta>
tag to page header and set refresh time as per your session time out + one or two seconds more (Google how to do this programatically instead of hardcoding of "Content=60")
2.In the page load check the Session avalible or not, if not
ModalPopup.Show()
Page will get refresh after session time out and your ModalPopup will appear.