The most common reason that I find this happen on self made PCs is an extra motherboard mounting post. What happens is the use of jacks or slots eventually wears off the paint and then it shorts out.
That's one scenario but what you have is generically called "The Dead PC." Troubleshooting varies a little but always involves the classic make it smaller work.
the other day I plugged in my headphone into the front socket which caused my pc to shutdown. now whenever I turn it on it does a short quiet beep before turning off again.

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