Backlights on LCD screens are just little skinny flurecent bulbs.. most screens have two.. one for the top, one for the bottom.
The most popular cause of failure of a backlight is a dead or dying high voltage board that powers those bulbs.. altho occasionally, the bulb itself will die.. but not often.
If you're a happy hacker modder type, it can be fixed pretty easy and inexpensively.. just buy a iMac with a cracked screen and a good backlight and swap HV boards, then resell broke iMac on eBay, and you'll likely have made the repair for FREE, or possibly MAKE money on the deal.