Question PC shut down automatically and not turning on.Help please ,!

Jul 14, 2019
I'm facing a strange issue on my pc.
Yesterday suddenly my pc turned off abruptly while it was on .
Then when I tried to turn it back again it's not turning on .No POST ,no beeps ,both CPU and PSU fans are not spinning .
In such cases first thing that comes into mind it could be the thermal shutdown .But in my case probably it's the problem .Cause I changed the thermal paste 2 weeks ago and it was working fine.Even before the PC shut down automatically I checked the temp and it was absolutely fine .To identify the problem things I did so far -
1) CMOS reset
2)Tested with a different PSU
  1. Checked the existing PSU by shorting the green wire and the adjacent one .The fans are spinning normally .
  2. Changed RAM slots.
None of them worked...So finally I removed the heatsink ..and saw some excess thermal paste I cleaned it entirely and reapplied a new noctua thermal paste .This time I was extremely cautious and used a very tiny pea sized paste.
Then reconnected other the PSU ..and pressed the power button the fans started spinning !!! The PC booted into BIOS successfully !!!
So I thought the problem has been fixed ! ..attached other components like Gpu , storage drives , another ram stick and closed the PC case.
Unfortunately again the same problem brought back ..when I'm pressing the power button nothing happens POST no beeps ..CPU PSU fans are not spinning .. entirely a dead system ..just like before .

Again I detached additional components like Gpu ..extra ram stick ,HDD ..and tried with bare min hardware ..CPU ,Mobo and RAM ..but no luck .At this point I'm completely lost.Can anyone help me regarding this strange problem.? Thanks in advanced. I'm sorry for this long post .
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