No Power to Laptop

Hello everybody, can anybody tell me what's wrong with my laptop, one day it works the next day dead. I have checked that the charger, and power supply from the mains works fine. But no power to Laptop, I have tried to remove the bottom part of the casing, but having taken out all the screws, hardrive, it would not come apart, so i stoped as I didn't want to break it. My first through was the transformer unit, at least that's what I think it's called (the part where you plug the mains in), I have replaced one of these on my desk top so i thought it would be easy to do, Any ideas Please?????
Jim