how can i fix my dell c640 laptop ?

The power may get to the laptop. In this case, it may be overheating. In this case, after some time, you'll be able to restart it, but it will shut down again.
Make sure that the air vents are free of any obstruction. Check that there is no dust buildup in the air duckts, on the CPU's heat sink and that all fans are working properly.
Remove any dust: A can of compressed air is great for that task. Replace any non-functional fan if needed.

Maybe there is actualy no power comming in.
Maybe you, or someone, accidently disconnected the charger. Maybe the charger's whire got damaged by passing under the leg of some furniture or under some rug.
Make sure that the whire from the charger is intact and not crushed, cut, or otherwise damaged. If the whire is intact, reconnect the charger.

Maybe you have a power failure or a breaker disjuncted in the house power box. In this case, you have other devices that also stopped working.
Chack what is connected to that circuit. Try to identify whatever may cause an overload and disconnect that. Maybe there are to many things connected to that circuit: Disconnect some devices and look if you can connect them to another circuit.
Reset the breaker.

Electro
December 2012