Chances are your keyboard backlight isn't even broke ^_^
FIRST: Identify
Make sure you have a Powerbook with a backlit keyboard, other wise why are you here.
SECOND: Test
Make sure it won't come on, if your pressing the button to turn it on (F10) and nothings happening, you are probably in a well lit room, in which case it won't come on. Even when you cover the light sensors. SO take it into a completely dark room, no natural light, then try it.
THIRD: Reset PMU
IF it stillllll doesn't work, turning it off, unplug the battery and power adapter then hold down the power button for 10 seconds, then reboot, when booted back up, it should say your clock and what ever has been reset, change the clock and date back to normal, then reboot again ( if you don't reboot some internet apps might be messed up until you do) THEN see if it works, if it still doesn't try these link:
also google is your friend.