I'd say the most likely cause is a failed electrolytic cap.
I have seen quite a few PSUs that will not work when turned on after being off some hours, but if left on for a while then turned off and on again, will start working. Every time I've done one of those it's been failed capacitors.
That's not a 100% guarantee, but definitely worth a try.
I did one last week with big can capacitors similar to those - when I took the old ones out, one read about the right value and another something less than a thousandth of what it was supposed to be.