Turn the laptop on and plug/connect the AC adapter. If the power lights/battery indicators are on then:
1. Get a flash light.
2. Shine the flash light at the screen. You may see what you normally see when the laptop is working fine.
If you actually see the images but it is incredibly dim to the point it seems there is no video then you have either a dead CCFL orinverter board. LCD matrices cannot produce their own light, so they require CCFL lamps or LEDs to provide the light to show the image clearly and brightly on the screen.
The best thing to do there is to order an inverter board from ebay using your laptop's model number then the keyboard "inverter" they should not cost much. Replacing the CCFL lamp is not recommended unless you have experience due to the possible mercury found inside. A full LCD replacement would be in order if the new inverter does not fix the problem.
If you notice there is still no image what so ever even flashing the fight at the screen, then connect an external monitor via VGA. If there is no display it is a possibility the new motherboard was or is also dead.