first you need, username_txth.Equals(username)
using == means they are the same object, not that the strings are the same value
to show the form again need to pass the Owner (this)
MenuForm form = new MenuForm();
form.Show(this);
this.Hide();
3. show form again on closed or exit application
protected override void OnClosed(EventArgs e)
{
base.OnClosed(e);
this.Owner.Show();
}
cheers