Hi NecroSonata,

Based on the description, issue could be due to Power supply failure or Motherboard failure. Have you tried checking with a different wall outlet and also a different power cable? Please follow the below mentioned steps to troubleshoot:

1. Try a different wall outlet, and try to power on the computer.
2. Reseat the power cord which is connected to the computer.
3. SWAP the power cord of the Monitor with the system power cord, and power on the computer.
4. Disconnect the power cable from the rear end of the computer.
5. Press and hold the power button for at least 15 seconds.
6. Connect the power cable and press the power button.
7. If issue persists, disconnect the power cable, and perform step # 5 again.
8. Refer the following link for the Dimension C521 service manual: http://bit.ly/1d5tq3u
9. Follow the instructions from the manual to remove the computer cover.
10. Remove and reconnect the 24-pin power connector on the motherboard.
11. Remove and reconnect the 4-pin power connector on the motherboard that is located next to memory modules and diagonally opposite to the 24-pin power connector.
12. Replace the computer cover and connect the power cable.
13. Press the power button and check if the computer power’s on.

Please post the results.

 NecroSonata

Suspect a power supply failure.

First check that all the power and data cables are properly connected, including the cables to the front panel.

If you are comfortable working around computers, you could try the following:

Note: The only 100% method of testing a PSU, is to install a known working power supply.

Unplug the cord from the power supply, hold the power button in for about 15/20 seconds, open the case, unplug the 24-pin power connector from the motherboard and jump the Green wire to one of the Black wires, reconnect the power cord and power the system on, if power supply, case fans and the hard drive run, then the PSU may be be good.

Note: Do not remove any wires from the plug, use a small piece of wire or a paper clip as a jumper.

Power supply checks out and the system still does not work, again remove the power cord, hold the power button in for several seconds to discharge the residue power, reconnect the 24-pin connector to the motherboard.

Remove all the PCI cards, the video card [if applicable], memory, all peripherals, disconnect the data and power cables to all the drives, check that the front panel cable is connected to motherboard, with nothing else connected to the system, reconnect the power cable and power the system on.

If there is no difference in the diagnostic lights, 'Beep' codes and/or the LED indication of the power button, it would appear that the motherboard has died and/or maybe the processor and the only method I know of checking a processor, is to install it a compatible working PC, or use a known working processor.

 The C521, like a lot of Dell's other older small form factor systems, support's a Dell proprietary power supply, due to it's shape and physical size.

http://www.discountelectronics.com/product?gclid=CNf5iab2nbgCFWVo7AodxSAAlw&product_id=2105&product_model=63

You may find a PSU for less on eBay.

Bev.