A long press of the power button should force a restart. 10 to 20 seconds
Good question, I would like to know this too. When in service mode I have stepped through various screens by pressing the Return/Lap Key. One of them turns the whole screen into a set of rectangles increasing in size from the centre outwards, which is kind of hypnotic if you stare at it and I guess is a test of the display. Another has the heading 'Backlight' and has five squares spaced around the perimeter, some of which can be switched from blank to black and the Backlight % value can be increased/decreased between 0-100. Very mysterious!
I also stuck in this service menu and cannot exit. Already tried many times pressing down and hold power button for while, but again service menu shows up.