My iMac turns off halfway through start up.
My iMac shuts down halfway through start up.
iMac 27″, macOS 10.13
Question marked as
First try Safe Mode, as per > How to use safe mode on your Mac - Apple Support
Next try all the steps, at > If your Mac starts up to a blank screen - Apple Support
If those do not help, then the iMac has a hardware issue and needs to be serviced.
For service help, go to > Get Support