If you have Nikon’s new flip-screen DSLR, the D5000, and it won’t switch on, it’s because Nikon didn’t make it properly. A component glitch is causing some D5000s to remain resolutely off, even with a full battery or when hooked up to the mains. From the official statement:
To be fair, these things do happen even to the most careful of manufacturers, and at least Nikon is dealing with it properly. Anyone with an affected camera can send it back and to have the offending component replaced. This will all be free of charge, including two-way shipping. If you’re camera is already broken, get in touch. The rest of you lucky D5000 owners should wait until July 23rd when Nikon will publish its serial number look up tool, which will be at the page linked below.
D5000 Service Advisory [Nikon]