Why did a broken computer turn on?

Question

Electronics

Due to a problem with the video card, after each power supply to the power supply, the computer turned on.
So, 4 days ago my uninterruptible power supply failed: turned off, turned on, turned off, did not have time to turn on (I turned off the power), with an interval of 0.5 - 1 second. I corrected the malfunction in the uninterruptible power supply (the button on it was turned off, but for some reason it was supplying power to the output).
I turn on the computer - the fans are spinning, but there is no POST signal from the tweeter, there is no USB power for the mouse, the keyboard does not respond, there is no video on the monitor. I have no idea what was broken, but I disconnected everything: video card, CPU, RAM and added one by one and monitored the status. Checked everything. I swapped the RAM, shorted the wires from the CMOS, took out the battery.
3 days have passed, I took the computer to the master. Surprisingly, he said that the computer is working and he honestly did not repair anything. Today I took it, the computer really works.
So the question is: if nothing was broken, then why didn't the computer turn on? And what should I do if this suddenly happens again?

Answer 1

Inspect the capacitors on everything. On the PSU, mother, video card. If there is swelling or damage - back to repair. It happens that after transportation they work normally for some time, so the computer turns on normally in the service.