You didn't say what kind of light this was.
I once had occasion to go on a service call for a similar problem. All of a sudden the light could not be turned off. Turned out to be that the lights were switched on the nuetral instead of the ungrounded conductor,and the ballast in the fixture had the windings go to ground. When the the property owner tryed turning the lights out nothing happened. After changing the ballast the lights worked on the switch again. I also took the time to correct the problem with the switched nuetral.
(Who says nobody calls an electrician when the lights are on ? )
