Session state can only be used when enableSessionState is set to true, either in a configuration file or in the Page directive- SharePoint2010

Complete Error Message

Session state can only be used when enableSessionState is set to true, either in a configuration file or in the Page directive. Please also make sure that System.Web.SessionStateModule or a custom session state module is included in the <configuration>\<system.web>\<httpModules> section in the application configuration.

Before you start your troubleshooting, make sure that the State Service service application is running. How you will check this?