Are there any signs of life at all when you try to power on? Is there a light on the AC adapter plug, and if so, what is it, and is it stable, i.e. does it flicker?
I'm betting on Ben's suggestion, because I often see people trying to use 45W adapters on PowerBook 15"s, but if it's not that, you might try moving the memory to the other slot, and testing one slot at a time by leaving a memory bank open and seeing if that makes a difference.
Also, try resetting the PMU by holding down the power button for 15 seconds until you (hopefully) see the sleep light flicker and then hear a tone.