Its very hard to say without knowing if there is a healthy electrical supply at the fan when the light switch is turned on. Its a process of elimination starting with the most obvious.
01 - Is there a fan Isolation switch
02 - How old is the fan
03 - Does the fan have humidity/timer/overun
04 - Has any work been done recently
05 - Does the fan start or show signs of working (motor sticking)
Matt
January 2009
Automatic Answer
We may have found other answers which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix switches and sockets.