This is a problem that's truly got me stumped, and I've been doing support for several decades.
I've set up my in-laws with a pc running Linux Mint 13 (so that they don't screw it up like they would a Windows pc). Except for a few issues it's been a great success! The pc is a hand-me-down Medion (with some NVidia video card) that's always been rock solid.
Today, they called me up saying there's no display, and sure enough -- after the splash screen the display actually turns off (as in, "the green light goes yellow"), and I can't even switch to a TTY to log in and check dmesg -- because the keyboard and mouse have no power! What?! They work just fine in the BIOS and have power through much of the boot process. I haven't yet tested with a PS/2 keyboard and mouse (I have some, but I didn't have them on hand at the time). Booting with no USB peripherals makes no difference: the display goes off, and plugging in a mouse doesn't power it up.
How can I diagnose this problem? What could possibly turn off power to USB and VGA after (or during) booting?
PS. When I say that the splash screen shows up, that's an approximation. It's never been able to show anything else than a couple of white bars across the display, but then the desktop comes up just fine in full resolution. I just never bothered to fix the splash screen.