Why does the computer not turn on, only the b / n fan spins once?

Question

Iron

The computer suffers from the following:
- it suddenly turned off,
- removed / disconnected all components from the case,
- left the motherboard, processor, processor cooler, 650w power supply,
- still does not turn on (even after a day of inactivity),
- only the fan spins and stops (and not every inclusion - a pause of several minutes is needed).
This is a typical scenario, treated by transfer, movement, light reassembly. Then it works for a while - or minutes, or a week - unpredictably (which is the worst).
How can you understand the reason for such shutdowns? Localize the problematic component? On what component to sin? Could this be a typical symptom?
In principle, all components are under warranty, but I have no idea what exactly to carry to the seller and what to say. (Self-collection.)

Answer 1

100% power supply tested by many years of experience.