Disconnect all peripherals and the power cord, remove the battery. Take a deep breath & count to 60 slowly. Reconnect the power cord and battery and see if the machine starts normally. If it does, you can start connecting all peripherals one at a time.
If this tactic does not work, disconnect the power cord and hold down the Power button for 5 seconds, then reconnect the power adapter and press the Power Button to turn on the computer.
Your last resort is to take the computer to an Apple store or an AASP. Whichever is more convenient for you.