My iMac 2011 using macOS Sierra 10.12 is not starting up and restarting, How do I fix it?
My iMac 2011 uses macOS Sierra 10.12 not starting up and restarting. How to fix this
[Re-Titled by Moderator]
Your computer is quite old and has likely suffered a hardware failure. However, before you go out and buy it's replacement (strongly recommended) please follow Apple's troubleshooting steps in:
If your Mac doesn't turn on - Apple Support
and
If your Mac doesn't start up all the way - Apple Support
Best of luck.