Hardware failure, if you're lucky you just have a lose wire somewhere. Could also be the inverter (pretty cheap part but labor will add a bit to it), or the backlight, or possibly the LCD itself. Need to bring it to a repair shop for testing/repair unless you know how to take apart the case and part of the screen.

If the laptop is more than a few years old, the only part worth repairing is the inverter. Anything else will cost close to a new laptop in parts and labor unless you have someone that knows how to work on laptops.