Blank screens are caused by a number of problems:
1. backlight broken - shine a bright light at an angle to the screen if a faint imaged is seen then it's either the backlight or the inverter that powers the backlight.
2. inverter - see above
3. cable (wires) from the motherboard to the screen
4. broken LCD
all of the above require a qualified tech to diagnose and/or fix. Often it costs more to fix than to buy a new laptop.