I don't know much about the DS, but I do know a lot about electronics. It could be a loose ribbon cable, but I doubt it. It sounds like your DS motherboard has been fried.
Static electricity is most of the time the main reason for this. Static electricity can be produced when you work with electronics on a carpet, or when you are wearing a sweater and it rubs against your shirt. Whenever I work on an electronics project I use antistatic gloves or an anti static wrist band, or a anti static keychain. This has never happens to me, but people have told me that they fried their computer RAM just from placing it on the carpet for a moment...
But, Tri-edge could be right, but I will still go with the fried theory... Most people just don't take the proper precautions with electronics (clean work space, anti static device, forces things, etc...), and they end up sad in the end...