Unplug it, press and hold the power button for 10 seconds. Plug it back in and give it a try.
This fix is called an SMC reset. Useful when you have one or more of these issues"
Indicators that your SMC might need to be reset
After you perform normal troubleshooting, these symptoms may indicate that an SMC reset could be necessary:
The computer fans run at high speed, though the computer is not experiencing heavy usage and is properly ventilated.
The keyboard backlight behaves incorrectly on Mac computers that have this feature.
The status indicator light (SIL) behaves incorrectly on Mac computers that have this feature.
Battery indicator lights, if present, behave incorrectly on Mac notebooks that have a non-removable battery.
The display backlight doesn't respond correctly to ambient light changes on Mac computers that have this feature.
The computer doesn't respond to the power button when pressed.
A Mac notebook computer doesn't respond properly when you close or open the lid.
The computer sleeps or shuts down unexpectedly.
The battery doesn't charge properly.
The MagSafe power adapter LED doesn't indicate the correct activity.
The computer is performing unusually slowly, though it isn't experiencing abnormally high CPU utilization.
Application icons may bounce in the Dock for an extended amount of time when opened.
Applications may not function correctly, or they may stop responding after being opened.
A computer that supports target display mode does not switch into or out of target display mode as expected, or it switches into or out of target display mode at unexpected times.
The illumination around the I/O ports on a Mac Pro (Late 2013) does not activate when you move the computer.