Fix Mac Won't Boot/Start after macOS High Sierra Update 2017

1. Reset SMC to fix Mac won't boot after High Sierra update

Sometimes, if the Mac computer cables are not connected properly, you may not be able to boot it normally. So if Mac cannot boot after a system update, you can reset SMC to fix Mac won't boot issue after macOS High Sierra update:

1. Shut down Mac computer and unplug all cables;
2. Wait for a few minutes and then plug the cables back;
3. Restart Mac computer to see if you can enter the system or not.

2. Force to reboot Mac and reset NVRAM to fix Mac won't boot issue

1. Press and hold Power button for a few minutes till Mac turns off;
2. Wait a few seconds and press Power button to turn on Mac;
3. Hold Command + Option + P + R keys while booting Mac to reset NVRAM;
4. Reboot computer again to see if the problem gets solved or not.