No power is normally caused by either a bad Mosfet or a bridged connection. In the traditional xbox 360, if you have no power due to a bad Mosfet (power regulator), it will turn on and red ring, reading codes 0001,0002, or 0003. I am not sure how slim error codes work so I couldn't say for sure the source of your problem.. but if you hadn't opened it before I would say check inside of the USB ports and make sure nothing in them is bridged.. also look for bridged connections on the motherboard. Other than that, you'd have to check each mosfet for correct power output and replace the ones that are bad and hopefully that would fix it.