TextBox1_TextChanged Index to fill all Columns and Row

A textbox does not have a selectedindex_changed ;-)

I suggest that you handle the keypress event. Check for 0x0D or 0x0A ( key).

Code like

private void textBox2_KeyPress(object sender, KeyPressEventArgs e)
{
    char x = e.KeyChar;
    if (x == 0x0A)
    {
        MessageBox.Show("0x0A", "KeyPress");
        e.Handled = true;
    }
    if (x == 0x0D)
    {
        MessageBox.Show("0x0D", "KeyPress");
        e.Handled = true;
    }
}

Instead of the messagebox, read data from your datasource (database?) and populate the fields.

Note that above is code for WinForms, not WebForms.