iMac won’t power on
I started my iMac and as it was booting it shut off and there is no power.
iMac 21.5", macOS 10.13
Please try the troubleshooting in What to do if your computer won't turn on . However if there is no power at all and no video then this sounds like a failed power supply. Please take the computer to your local Apple Store or independent Apple Authorized Service Provider to be diagnosed and repaired.