how to validate Session....

Session["username"] = txtUsername.Text;

if(Session["username"] == null)
{
   //Redirect to login page.
}
else
{
   //Redirect to home page
}

protected void btnImage_Click(object sender, ImageClickEventArgs e)
    {
        try
        {
            adp = new SqlDataAdapter("Select UserName from AdminLogin where UserName='" + txtusername.Text + "' and Password='" + txtpassword.Text + "'", conn);
            tbl = new DataTable();
            adp.Fill(tbl);
            if (tbl.Rows.Count == 0)
            {
                cls.MsgBox("Invalid UserName or Password...Login Failed", this.Page);
                txtpassword.Focus();
                return;
            }
            Session["User"] = tbl.Rows[0]["UserName"].ToString();
            Session["SessionId"] = "valid";
            Server.Transfer("Default.aspx");
        }
        catch (Exception ex)
        {
            cls.MsgBox(ex.Message, this.Page);
        }
    }