XPS 8940, turns on but no signal

The PSU on the back, what color is the LED?

Power Button, blinks with colors and that would be a clue to the problem.

2 amber, 1 white (2,1) for CPU problems, 2,3 for no RAM, 2,4 for RAM failure, or 3,2 for video/PCI issues, indicating specific failures. To find your code, observe the pattern (e.g., 2 amber flashes, then 4 white flashes), then check the Dell manual or support site for the corresponding hardware fault and troubleshooting steps like reseating RAM or checking the battery.