Check power connections
Make sure that your power cable is undamaged and plugged securely into your Mac and a working electrical outlet.
If your Mac has no built-in display, make sure that your external display is connected to power, connected to your Mac, and turned on.
Press and hold the power button
Press and hold the power button on your Mac for about 10 seconds. (Every Mac has a power button. On laptop computers that have Touch ID, press and hold Touch ID.)
Then press and release the power button normally.
You could have an issue with one or more accessories or their cables. Unplug all accessories from your Mac, including printers, drives, USB hubs, and mobile devices. Then repeat step 2, above.
If you see something on your screen
If your screen shows a flashing question mark, circle with a line through it, or anything else, your Mac is turned on but isn't starting up:
If you see nothing on your screen
If your Mac seems to be turned on but the screen remains blank, the issue could be software related:
Reset the SMC (Intel-based Mac only)
If you're using an older, Intel-based Mac, resetting the SMC might help:
If your Mac still doesn't turn on, contact Apple Support.