iMac shuts down upon reboot

[Sumrday]

my computer was doing automatic system update and has gotten stuck. I now have a white screen with spinning wheel that shuts down after 20 seconds. Unable to reset or access menu.

Anyone know how to fix this.

 

[Buerkletucson]

my computer was doing automatic system update and has gotten stuck. I now have a white screen with spinning wheel that shuts down after 20 seconds. Unable to reset or access menu.

Anyone know how to fix this.

System restore from Time Machine backup...

https://support.apple.com/en-us/HT201314