Power button stuck

I'd say remove extended memory, Airport, and battery to avoid complications, and then connect via AC power and see if you can reset the PMU by holding the power button in for 10 seconds. If that doesn't work, leave it sitting unplugged overnight and try again in the morning (letting the computer discharge with no power source including battery can sometimes clear up problems that cause it to not power on). I'd also try to verify that your AC adapter is good on another machine (and that both parts of it are firmly connected together). Also, when you plug in the AC, is the light on the AC adapter plug a steady color as it was while it was working, or does it flicker, etc.?

Generally power buttons don't go bad, and usually when a machine won't power on, the problem is something deeper, and the power button itself is just the messenger telling you the bad news. You can replace it, and 1 in 50 times you might find that that is the issue, but far more often than not it's going to be something else.