This is just a thought.
Have you tested the on/off switch?
Mine developed what is known as a 'Dry joint' on the solder connections at the on/off switch causing a current flow resistance.
The switch itself tested ok.
On advice of electronics engineer re-melted the existing feed in solder joints [un plug from socket!! you'd be supprised how many forget It's still plugged in and live, and boom!!!!.] adding solder if necessary. Re-assmbled and switch on computer and if everyithing came back to life. Job done.
Try it it's a thought, however if you found what cause the problem then post the prob and solution so others can read who may have same or similar probs.
Another
Test power cable from socket to ensure there is no break in the conductors
Bob, December 2008