So, my friend's computer (dell xps 400 desktop) just turned itself off the other night, and refused to be turned back on. When we tried to turn it on, we got a flashing yellow light on the power button and it made no noise. Someone suggested that I remove the hard drive and turn it on again, as that if it were the hard drive that was bad, we'd get a message on the monitor about not being able to locate the operating system. We got no such message, but now the computer would make startup noises and the power button was green. When we put the hard drive back in, it still powered up green and made noise, but still nothing on the monitor.

What does this sound like you you guys?

The machine has more serious issues, most likely power supply problem (be careful on a Dell as they may be uniquely wired which means put a normal power supply on it and you'll kill it) or a board problem.