It is an effective method to troubleshoot hardware misbehaving or slow performance.
If your Mac still won’t boot after Sierra update, you can reset SMC (System Management Controller) to fix the failing Mac startup. Reset SMC If macOS Sierra Won’t Boot after Updating You can also learn how to fix MacBook Pro shuts down randomly or fix macOS Sierra 10.12.1 Update Stuck Error. Step 3Release those keys when you hear the startup sound. Step 2Hold and press Command, Option, P and R keys simultaneously during Mac startup. Step 1Power off and on your Mac computer. So you can fix macOS Sierra won’t boot after updating via this method. Resetting PRAM/NVRAM will not erase your Mac data. Generally speaking, PRAM or NVRAM can save data that persists while the Mac is shut off. NVRAM is short for non-volatile random-access memory. Reset PRAM/NVRAM When Mac Won’t Boot after Updating
