It might have happened that when you are in a public or work toilet the light turns off because of the motion sensor, and as much as you wave your hands you cannot make it on again.
Is there an easy way to turn it back on again?
I'll assume the light won't turn on again because the sensor can't spot your hands moving within the stall. The door and the stall walls block the sensor's "view". I can think of a few options:
I recently found out that many toilet motion sensors are based on a radar principle, and they use microwave radiation for that purpose.
Thus, a very easy way to trick the motion sensor is to pick your mobile phone and start making a call. You don't have actually to make a call, just let it start making the connection as the microwave radiation will immediately interfere with the motion sensor and turn the lights back on.
If the motion detector is actually next to your cubicle, then you are in luck - otherwise you might have to cross a few doors to get it light up again (happened to me, not funny).