Try removing the battery. I've seen near-dead batteries keep a machine in sleep mode and not let it wake up. After removing the battery and AC, connect the AC again, and immediately reset the PMU and PRAM.

Does the machine chime when you power it on? If it does, it may be a screen-related issue (check to see if there is a dark image on the screen), and if it doesn't, I would reseat the RAM and test for bad memory slots.