I have a Dell XPS 600 desktop computer. I cannot get the power to stay on. None of the diagnostic lights light up. I have checked all of the connections and everything is secure.
Here's what to try: Disconnect everything from the motherboard except the CPU and memory. You may leave the video connected to see what, if any, error message you might get.
You didn't say if you got any beeps on startup. They can tell you what might be the problem.
If this doesn't work, remove the memory modules. The power should stay on but you'll probably get either a steady beep or a bunch of little beeps. (I don't have the Dell error codes close at hand at the moment).
If this doesn't work, you have narrowed the problem to one of three things: The CPU, the motherboard, or the power supply.
You didn't mention just what happens when you turn it on--just that it won't stay on. Error messages are helpful if you get any on the screen as are any beep tones.