HP laptop charger light on, but won't turn on.
I have checked; charger - Good
19.6 volt path - Good thru all sub power supplies.
3.3 volt - Present at IO.
Tried Mobo in & out with nothing connected but old, then new power button - Dead.
Any one of many component failures could cause the unit not to turn on. You have checked some of the obvious items but there is no way for you to check the CPU if, for instance, it had failed hard. I had an HP laptop within the last year that would also not turned on. It was a bad power switch but replacing that was not feasible for me to do (and I have a pretty well equipped home lab). I could tell it was the power switch because it was no longer "clicky" feeling. Sent it back to HP for repair. Was your failure just out of the blue or had you had the machine apart?