If you see the following error message when trying to connect to your portal:
Server Error in '/' Application.
Unable to make the session state request to the session state server. Please ensure that the ASP.NET State service is started and that the client and server ports are the same. If the server is on a remote machine, please ensure that it accepts remote requests by checking the value of HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\aspnet_state\Parameters\AllowRemoteConnection. If the server is on the local machine, and if the before mentioned registry value does not exist or is set to 0, then the state server connection string must use either 'localhost' or '127.0.0.1' as the server name.
It is likely that your ASP.NET state service is not started. Log into the Web Server, launch the Services menu, and check to see if this service has been started. Also ensure that it is set to "Automatic", rather than "Manual". This will prevent the problem from returning when the server reboots.