object reference not set to an instance of an object

[C# Programming]

List lstusers = new List();
lstusers = AMSManagement.GetAllUsers();
var lnquser = from u in lstusers

show error here plz help me!!!
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where u.User_ID == int.Parse(Session["UserID"].ToString()) && u.Pass_word == txtopwd.Text.Trim()
select u;
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if (lnquser.ToList().Count > 0)
{
users u = new users();
u.User_ID = int.Parse(Session["UserID"].ToString());
u.Pass_word = txtcnpwd.Text.Trim();
AMSManagement.Changepassword(u);
lblmsg.Text = "Password Changed Successfully!";
}