Possible dead Xbox mainboard after faulty power supply

My xbox has died after a faulty power supply. Im not sure fully what has gone wrong with it. After replacing my power supply, the basic disc drive functions work (disk intake and eject as well as corresponding chime) but nothing else works. The xbox doesnt boot at all, no start up chime or lights. After full disassembly, there are no signs of blown components (burnt regulators, etc.) and no special smoke/smell. Testing continuity/resistance across the mainboard power connector, the resistance was into the meg-ohms indicating no shorts in a power-off state at least. Any ideas as to what might be wrong?