session Object reference not set to an instance of an object?

HI...........

 I have Web page in that 'm going select a Project Names in Drop down list, when button click it display the Report in Gridview(access useing stored procdure from database),  After  1 hour or 2 uour in Same page I'm going to select another Project in Dropdown list it show  session Object reference not set to an instance of an object.
i need Permanent Suggestion for this please help me.......

thanks in Advance

Santhosh