You’ve lost something called the “Title bar.”
To get it back:
- Press and hold down the Alt key, then hit the spacebar, then the M key.
- Now, look on your keyboard for a grouping of keys with arrows on them. Hit the key that points down a couple times and you should see the window has moved down.
- Once you’ve got the window down far enough to see the title bar, click anywhere inside the window.
You should now be able to click on the title bar and drag the window wherever you want it, or click one of the buttons that minimizes, maximizes or closes the window.