Default SessionState mode of Asp.Net application is "
inproc".
This mode of session state management is as same as Classic Asp State Management, where session state gets maintained in the running process on the same Web Server/Machine. Whenever this process gets recycled by IIS, your web application looses it's session state information/data.
Have a look at below link, for more information on this.
http://blogs.iis.net/webtopics/archive/2009/07/21/in-proc-session-state-management.aspx
For more information on "
ASP.NET Session State" refer this.
http://msdn.microsoft.com/en-us/library/ms972429.aspx