How do I display a error message if login fail for windows form-Collection of common programming errors

I have a form with username, password and login button. I have three tables called doctor, nurse and admin.

I want to display error message if login is unsuccessful when button is clicked.

Below is my login button code. I tried putting in codes in the catch (SqlException ex) recommended by another person but it has no effect when the login button is pressed.

private void btnLogin_Click(object sender, EventArgs e)
{
    //retrieve connection information info from App.config
    string strConnectionString = ConfigurationManager.ConnectionStrings["sacpConnection"].ConnectionString;
    //STEP 1: Create connection
    SqlConnection myConnect = new SqlConnection(strConnectionString);
    //STEP 2: Create command
    string strCommandtext = "SELECT dUsername, dPassword from DOCTOR";
    // Add a WHERE Clause to SQL statement
    strCommandtext += " WHERE dUsername=@dname AND dPassword=@dpwd;";
    strCommandtext += "SELECT nUsername, nPassword from NURSE WHERE nUsername=@nname AND nPassword=@npwd;";
    strCommandtext += "SELECT windowsUsername, windowsPassword from WINDOWSADMIN WHERE windowsUsername=@aname AND windowsPassword=@apwd";
    SqlCommand cmd = new SqlCommand(strCommandtext, myConnect);
    cmd.Parameters.AddWithValue("@dname", textUsername.Text);
    cmd.Parameters.AddWithValue("@dpwd", txtPassword.Text);
    cmd.Parameters.AddWithValue("@nname", textUsername.Text);
    cmd.Parameters.AddWithValue("@npwd", txtPassword.Text);
    cmd.Parameters.AddWithValue("@aname", textUsername.Text);
    cmd.Parameters.AddWithValue("@apwd", txtPassword.Text);


    try
    {
        // STEP 3: open connection and retrieve data by calling ExecuteReader
        myConnect.Open();
        // STEP 4: Access Data
        SqlDataReader reader = cmd.ExecuteReader();


        while (reader.Read()) //For Doctor
        {
            if (MessageBox.Show("Login Successful") == DialogResult.OK)
            {
                timer1.Enabled = true;
            } 
        } 
        reader.NextResult();
        while (reader.Read()) //For Nurse
        {
            if (MessageBox.Show("Login Successful") == DialogResult.OK)
            {
                timer2.Enabled = true;
            }
        }

        reader.NextResult();
        while (reader.Read()) //For Admin
        {
            if (MessageBox.Show("Login Successful") == DialogResult.OK)
            {
                timer3.Enabled = true;
            }
        }



        //STEP 5: close connection
        reader.Close();
    }
    catch (SqlException ex)
    {
        string message = ex.Message;
        string caption = "Error Detected in Input";
        MessageBoxButtons buttons = MessageBoxButtons.YesNo;
        DialogResult result;

        // Displays the MessageBox.

        result = MessageBox.Show(message, caption, buttons);
    }
    finally
    {
        //STEP 5: close connection
        myConnect.Close();
    }
}