Try resetting the SMC controller. Turn it off, leave the MagSafe adapter plugged in. Hold down Option + Control + Shift + Power for 10 seconds. Observe the green/amber light in the MagSafe connector: typically it will change state to indicate a successful SMC reset. Then try powering on normally.
Remove the two long screws along the back edge, and the shorter screw in the middle. Torx T-8. Pry up the rubber feet, and unscrew the three Phillips screws that hold the feet in place. Putting the silver collars back requires a little bit of fiddling to get them oriented correctly so they sit flush with the bottom case.
Step 10 should come after step 15.
Remove the two long screws along the back edge, and the shorter screw in the middle. Torx T-8. Pry up the rubber feet, and unscrew the three Phillips screws that hold the feet in place. Putting the silver collars back requires a little bit of fiddling to get them oriented correctly so they sit flush with the bottom case.