First, make sure that the status LED is turning on when the power switch in the back it turned on. It should be red. If you are not getting a light, then there are a few places to check.
- First make sure that it is not an issue with the power supply. If one of the cables in or out of the power supply is not properly plugged in or is damaged, then the PlayStation will not get any power, and certainly, the power buttons will not work. This seems like the likely solution, since you cannot turn on the console with a remote either.
If the power supply checks out, then it could be the power board.
- If you have an early model (CECHA-CECHE) then the I/O contacts are on a separate board. Make sure that the cable between the I/O board and the Motherboard is both intact and properly seated. On one end, there is a typical ZIF connector, but on the other end, there is no connector, and the cable must be delicately thrust into the connection. Make sure that this connection is both straight and all the way in.
- Again, if you have an early model, you may have cooked the I/O board somehow. This is rather unlikely, but not impossible. It would have to be replaced.
- Irregardless of the model, make sure that the contacts are free of gunk or corrosion. If you have handled the PS3 motherboard a lot, then it is possible that there is buildup of oils and other stuff that would keep the button from registering a contact. It works on a capacitive discharge system. You can actually power the system on and eject disks with the cover off by simply touching the appropriate contact spring.
It is also possible that the motherboard bought the farm in all of that labor with the YLOD, which would certainly prevent the console from turning on.
So, as a recap: make sure that the power supply is alright, then check the other areas.
Good luck, and let us know how it works out.