Debugging a multilayer PCB?
If you have a current-limited bench supply, set it to 2 or 3 amps, and connect it between power and ground on the bare board. (Better: start with 100 mA and ramp it up to 2 or 3 A if it doesn't cook off the short)

Now probe with a reasonable voltmeter and see what part of the power net is closest to the ground potential. Your short is somewhere near there.