If your Mac screen goes black
If your screen goes black, your Mac or display may have gone to sleep. If you have a portable computer, it may be out of power.
Your computer or display may have gone to sleep
If your screen goes black, and then comes back on when you press a key or click the mouse or trackpad button, your Mac or display has gone to sleep. To save energy, your Mac and display go to sleep automatically when they’re not in use.
You can set your Mac and display to sleep at different times. Because the display wakes up faster than the rest of the Mac, you should set it to go to sleep before the Mac. See Set sleep and wake times for your Mac.
Your portable computer may be out of power
If your portable computer’s screen goes black, but is not in sleep, it may be out of power.
Plug the power adapter into the computer and into a power outlet.
Unplug any devices that are connected to your computer, except the power adapter.
Hold down the Command and Control keys while you press the power button to restart the computer.
Let the battery charge to at least 10 percent before plugging in any external devices and resuming your work.
The battery icon in the menu bar shows that the battery is charging 
. When charging is complete, the icon shows that the battery is full 
.