It doesn’t start up at all!
First of all, check that the Mac is connected to a power supply, and that the power cable is secure. Then you should check that the power socket is switched on and working. (Test it with a lamp!)
Next, eliminate the possibility that it’s a display problem. Is there a startup chime? Can you hear the fan? Is the sleep LED lit? Does ¬ light up when you press it? If the answer to any of those questions is yes, it may be a problem with the display. If it’s an external monitor, check its connection. If that’s okay, try connecting a different display. If the answer to all of those questions is no, try resetting the system management controller (SMC). Shut down, press (left side) ß+å+≈, and press the power button to start up. Still no joy? Get thee to a Genius Bar.