Sony VAIO PCG-FX340 Doesn't Boot

I was given this laptop to fix from my teacher. She said that she replaced the hard drive and the screen doesn't turn on now. She had to take it apart considerably since the HD is under the KB. I have taken it apart completly and attempted to resteat RAM/Processor/Heatsink but nothing has worked. All it does is turn on and the power LED stays on. Then it turns off normally if I hold down the power button. Hooking it up to an external monitor does nothing.