No boot. Green light on PSU blinks when 24 pin cable plugged into motherboard. No lights on mobo

[Shoreideas]

PSU green light solid when no components plugged in, but blinks when motherboard 24pin is plugged in.  No monitor or keyboard or mouse connected.  No boot. No beeps. No lights on motherboard at all.   System was being used when it suddenly turned off. 

[savvy2]

first off rule one (techs know A+) do not hot swap any thing inside any PC. ever.

or boom happens blow things up you will.

AC cord pulled from wall first. the push power button 3 times to discharge all caps inside the PC. (...

PC failed

the correct steps on a dead PC is , leave the 24 and 4 power cables alone . lets make a list, or this.

1. test the RTC coin cell battery. first ,always, below 2.9vdc is NO GOOD.  11 year old is NO GOOD
2. do the hard cold reset first. this can if lucky undo the latch you cause hot swapping parts inside the PC.
3. The hard reset all cures (again if lucky) power surge or minor lightning hits and chip latchup self preservation.
4. then see if Esc key works  and screens BIOS can be seen now.  after above done.
5. the PC power button must glow solid not blink turned on , if not do step1,2,3,. 
6. The PSU can be bad, and all 24 pin PSU can be bench tested, (basics) not load bank testing like I can do.
7. a real shop has  spare PSU to try, after all if it is bad there are no other steps to  do, (meter it sure) and not cause more damage, for sure hotswapping parts very bad idea.