Dear Rowan,
The first button in the top right hand corner of the panel means minimizing/muximizing the panel.
The second button in the top right hand corner of the panel means hiding/showing the panel. If you select it, the panel will hide and it will appear only when you move the mouse cursor to the hidden area.
The third button in the top right hand corner of the panel means closing the panel. Probably you clicked on it and the panel closed. To display it again you need to go to menu View, select option Panels and activate it again.
Regards,
Serge