see if holding down the power button and plugging in the receiver will power it on. if it does and seems to operate normally when the power button is continuously held down (check and make sure you can still switch sources and whatnot) then it's as easy as shorting out the power button.
if not, you may have to research into using a capacitor to momentarily "short" the power button on. (basically wired across the button contacts, a capacitor acts like a dead short until it charges up, then it appears "open")