The next step would be to isolate the source of the power failure by disconnecting parts from the PSU, see the Dell knowledge base article Resolve No Power, No POST, No Boot or No Video issues with your Dell Computer.
If the BIST LED does not light up, then the power supply is not able to deliver power to the system board and could be caused by any component that is connected to the power supply or the power supply itself. If the integrated self-test LED turns on, then the power supply unit can deliver power to the system board. If the fan is not spinning up properly this should be considered a BIST failure even if the LED lights up. The light should be solid on not flickering or flashing, and the power supply fan should also turn on to speed. Press the BIST button on the computer and check whether the LED lights up. How to run the self-test using the button on the PSU 
To learn more, see the Dell knowledge base article How to Run a Power Supply Unit Self Test On a Dell Desktop or All-in-One Computer.
NOTE: Certain older models of Dell Inspiron, XPS, and Vostro desktop computers may not be equipped with a PSU-integrated self-test.
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