[Solved] ASPX Page gets blank when displaying alert message box

My page goes blank when check box is checked or unchecked. Please i don't want it to go blank. It is after you click ok in the alert box that the page returns

protected void OnChckedChanged(object sender, EventArgs e)
{
    CheckBox chk = (CheckBox)sender;
    GridViewRow row = (GridViewRow)chk.NamingContainer;
 
    string number = row.Cells[2].Text;
    string pid = Label27.Text;
 
    string constring = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constring))
    {
        string acceptedCylinder = GetAcceptedCylinder(pid);
        if (chk.Checked)
        {
            if (acceptedCylinder == "[]")
            {
                using (SqlCommand cmd = new SqlCommand("UPDATE issuesheet SET cylinders = @cylinders where pid='" + pid + "'", con))
                {
                    cmd.Parameters.AddWithValue("@cylinders", "['" + number.Trim() + "']");
                    con.Open();
                    cmd.ExecuteNonQuery();
                    con.Close();
                }
                ScriptManager.RegisterClientScriptBlock(this, this.GetType(), "alertMessage", "alert('Record inserted successfully.')", true);
            }
            else
            {
                List<string> cylinderNo = acceptedCylinder.Replace("[", "").Replace("]", "").Replace("'", "").Split(',').ToList();
                cylinderNo.Add(number);
                using (SqlCommand cmd = new SqlCommand("UPDATE issuesheet SET cylinders = @cylinders where pid='" + pid + "'", con))
                {
                    cmd.Parameters.AddWithValue("@cylinders", "['" + string.Join("','", cylinderNo.Distinct()) + "']");
                    con.Open();
                    cmd.ExecuteNonQuery();
                    con.Close();
                }
                ScriptManager.RegisterClientScriptBlock(this, this.GetType(), "alertMessage", "alert('Record updated successfully.')", true);
            }
        }
        else
        {
            if (!string.IsNullOrEmpty(acceptedCylinder))
            {
                List<string> cylinderNo = acceptedCylinder.Replace("[", "").Replace("]", "").Replace("'", "").Split(',').ToList();
                cylinderNo.Remove(number);
                using (SqlCommand cmd = new SqlCommand("UPDATE issuesheet SET cylinders = @cylinders where pid='" + pid + "' ", con))
                {
                    cmd.Parameters.AddWithValue("@cylinders", cylinderNo.Distinct().Count() > 0 ? "['" + string.Join("','", cylinderNo.Distinct()) + "']" : "[]");
                    con.Open();
                    cmd.ExecuteNonQuery();
                    con.Close();
                }
                ScriptManager.RegisterClientScriptBlock(this, this.GetType(), "alertMessage", "alert('Record Deleted successfully')", true);
            }
        }
    }
}

Please help