Object reference not set to an instance of an object error

first read and think what is the error and thanks to microsoft to provide such a message
"Object reference not set to an instance of an object" means your object's instance is not made and you are trying to perform operation on it just first create your instance :) and you will get the answer

string strUserSanctionLevel ="";
if(Session["UserSanctionLevel"]!=null)
{

strUserSanctionLevel = Session["UserSanctionLevel"].ToString();
}

You are trying to insert value in null variable.
First Initialize 
String strUserSanctionLevel=new String();
then write
strUserSanctionLevel = Session["UserSanctionLevel"].ToString();


or Session["UserSanctionLevel"] is null so use Convert.ToString(Session["UserSanctionLevel"]) instead of direct ToString();