For Lightroom 1-4

Lightroom has several screen modes. You can cycle through them by pressing the F key. Two of the modes (Full Screen and Full Screen with Menu) hide the minimize, maximize and close buttons at the top of the application window. You’d be surprised how many people “lose” them and have trouble getting out of Lightroom.

So, if yours are missing pressing F will cycle you back to Standard Screen Mode eventually or you can jump right to it by pressing Cmd/Ctrl+Option/Alt+F.

Lightroom 5 and up

Lightroom 5 still has several screen modes, but Adobe changed up the keyboard shortcuts a bit. Now just pressing F takes you to the new true full screen mode, where just the selected photo fills the screen. Press F again to go back to where you were. To cycle through the other screen modes you need to hold the Shift key and press F.