PC does not start after turning off the back, why?

Question

Computers

It was necessary to transfer files from a 1TB disk to a 2TB one. Used Ubuntu 16.04.7 Live CD. Rebooted, he waited, Ctrl+Alt+Del pressed, waited.
I cut it out through the button on the back, launched it and ... It does not load. Even the fans don't start. There is power (the light on the motherboard is on), I cut down the disks, the flash drive too, I tried it only with a flash drive, only with a disk. Doesn't matter.

What to do?

Answer 1

There is another stupid option: while shamanizing with disks inside the case, I disconnected the wires going to the power button (Power Switch) from the mother.
In general, this situation means one of three things: either there are problems with the power circuit on the motherboard (usually swollen conduits), or the PSU has stopped producing enough power to start, or a short circuit occurs somewhere. The first two are checked, as mentioned above, with a minimum load (when only the mother is connected to the PSU, and everything should be removed from it, up to the video card and memory, but leave the processor).
A short circuit in the PSU is checked by disconnecting everything from it in general and shorting the green wire with any black one. If after that the fan on the PSU starts up, it means that at least there is no "shorty" in it.

Answer 2

Start testing with the power supply. When it was turned on, the capacitors held a charge, they turned off, they were discharged and could no longer be charged.