as a last resort
long-press the power button
release and press again

If your Mac doesn't turn on or start up

If your Mac won't turn on, or it turns on but doesn't finish starting up, try these solutions.

Check for power-related issues

Press the power button on your Mac (every Mac has a power button), then check for signs of power such as these:

Sounds, such as from a fan, hard disk, or other startup sound
Lights, such as from a backlit keyboard, Caps Lock key, or sleep indicator light

If your Mac has no signs of power, start by making sure that it's receiving power:

Make sure that your power cable is undamaged and securely plugged into your Mac and a working electrical outlet. If you're not sure about the outlet, test it with a lamp or other device.
If you're using a Mac notebook, make sure that you're using the correct power cable and adapter. Leave them plugged in long enough to provide a usable battery charge. Learn how to troubleshoot USB-C power adapters or MagSafe power adapters.
If you're using an external display, make sure that it's turned on and isn't experiencing any other display issues.
Make sure that your display's brightness is turned up. Mac notebook computers have a Brightness Up key on the built-in keyboard.

Then try these solutions, whether or not your Mac has signs of power:

Press and hold the power button for 10 seconds, then press it again.
Unplug all accessories from your Mac, including printers, drives, USB hubs, and mobile devices. (You could have an issue with one or more of those devices or their cables.) Once more, press and hold the power button for 10 seconds, then press it again.
Reset the System Management Controller (SMC).

If your Mac turns on but doesn't finish starting up, you might be able take steps based on the screen you see on your display.
If you recently installed memory or a hard disk (or SSD), make sure that it's compatible and installed correctly. If possible, remove it and test with the original memory or disk.