Shut-off while compting. Now it won't power up.I

that sounds like total power failure. likely a diode or rectifier on the motherboard has blown out, and does not allow the charger to charge the machine or power it. You can take it to a shop where they will try another power adapter. If that works, you are in luck, because a universal power adapter is usually not more than 50 bucks. You might also be able to get a used power adapter for that unit online for about 25 - 40 bucks. If however, the computer shop cannot get it powered up, you are better off replacing that unit. the c640 was fine in its day, but now there are d600's for 250 bucks online, and they are way better. You can also get one (not exactly NEW) off the dell outlet site Outlet.Dell.Com. They have lots of refurbished and "used as a display" computers that work just fine, and come with all the same guarantees as a new one off the line.