Create new form

[C# Programming]

Hi,

I have a form that gets data from DB,
i created an event when the form deactivate the current form gets this.Hide() and a new form is open.

my problem if how i can reopen the first window?
if i write-

FirstWindow frmFirst = new FirstWindow();
frmFirst.Show();

a new form is open, but not the first form (the one that i turn to Hide()).

How can i solve this????
:(