There appear to be 2 or 3 different problems in this thread - buttons missing altogether (press F once or twice), or buttons visible but not working. So which problem are you having sdeleng, and which LR version and OS?
Hi. ---> Answer at bottom.
LR 5.0 / Mac OS X 10.8.4 / iMac 27"
The LR window is zoomed out, and I can't change the window size.
The Red, Yellow, Green buttons and the gray application top bar is missing.
CMD-M minimizes the window, but does not help change the window size.
I can't grab the top, left, right, bottom or corners of the window. The cursor does change, but I am unable to resize the window.
The menu option Window / Zoom does nothing.
once, twice works, but the top bar does not return.
Restarting the Mac does not help.
Relaunching LR does not help.
I quit all other applications.
When I activate Mission Control, I can see the red/yellow/green buttons and the gray top application bar.
As I was about to delete the preference files I tried this and it worked!
[shift] F