Won't boot - at all

I have a MacBook Mid-2009 2.1 GHz.

I've tried booting (with power adapter connected) from the internal and a bootable external HD, also a Leopard DVD. In each case, the white light on front of case comes on, it hums a bit, a click or two, then nothing (but the light stays on). Never a startup chime, and the display remains black.

User reports that it worked fine until... it wouldn't start. I've removed the HD, put it in a Voyager enclosure and it mounts to another Mac no problem (but I haven't booted another machine from it). I suspect some sort of power issue, but basically don't know what to try next, and am looking for suggestions.