Do you have a spare working 24/4 pin psu to test 8920?  It can be any power or form factor.  This is one way to prove if stock psu died completely. you can uninstall the stock psu and try a paper clip test to see if the psu can turn on at all by itself. if you have a spare working motherboard that takes 24/4 pin, you may also use that to test the 8920 stock psu.  Uninstall psu from 8920 and connect to test motherboard to see if it can be turned on.  
I have an XPS 8920, and with a bad weather warning coming, I shut down the machine (normal windows shutdown). When I went to turn it back on again in the morning, nothing - nada. No lights, no fan, no power. I've done the following: 1. Replaced power cord and plugged directly into a known, working outlet 2. Replaced the CMOS battery (tried with 2 new batteries) 3. Unplugged the PSU from the motherboard, graphics, IDE drive and a few other cables that I could reach. The only thing I couldn't easily unplug was the cable going to the CPU cooler. Plugged the power cord back into the PSU and pressed the BIST button - no response. No light, no fan, etc. Is there anything else I can try to isolate whether or not it's the PSU? I'd like to be fairly certain before I order (pretty price) and then wait (4-5 days). Any help with this would be great.   
XPS 8920, normal shutdown, now no power on?