My IMAC powers on, shows the apple logo, the progress bar gets halfway through and then I hear a click and the computer powers off.
iMac 21.5″, macOS 10.13
Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?
If that doesn't work hold alt key at startup, how long will it sit on the Boot Choice screen?
For what it's worth, give this a try. After it powers off, remove the power cord for an hour or so. A couple months ago my 27" 19,1 iMac acted weird like that. After making an appointment with a local Apple Store Tech and driving there, we used a store power cord and turned it on. Booted up fine. Used my power cord and worked fine. Back home I went. The Tech mentioned that happens sometime and the machine needs to be completely disconnected from power for awhile.
Disconnect all devices also.
My iMAC powers on, shows the apple logo, the progress bar gets halfway through and then I hear a click and it powers off
