Unfortunately ASP.NET doesn't provide a nice out of the box solution for handling the session timeout gracefully. Everything expires behind the scenes and your user is left unaware of what has happened. Additional problems can arise if the user abandoned their browser in a state that you didn't code for, which can result in errors or exceptions taking place.
With this in mind I like to put a Session Expired page in place in my applications and pro-actively send the user there when their session ends....
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Aaron Schnieder
http://www.churchofficeonline.com