You can do that on this way:
(I'm not sure for the quality, but it works for me)
private void button1_Click(object sender, EventArgs e)
{
Form2 newForm = new Form2(this);
newForm.Show();
this.Hide();
}
Form parentForm;
public Form2(Form form)
{
InitializeComponent();
this.parentForm = form;
}
private void Form2_FormClosing(object sender, EventArgs e)
{
this.parentForm.Visible = true;
}