My iMac is normally fine but when I changed the start up disc today from High Sierra to Sierra it refused to start. It chimes and the light grey screen with progress bar opens but after a few seconds it switches off. It's the same when trying to start in safe mode. I thought it might be heat but it's the same even when cold.
It's beginning to sound like hardware failure. See if it can boot into Target Disk Mode . If it can then the problem lies in the motherboard or other components. It you can get into TDM you can recovery your user data for transferring to a new Mac.
See if you can boot into the Recovery volume in Sierra (boot with the Command + R keys held down - Intel Macs) or (How to Boot an M1 Mac into Recovery Mode).
If successful select Disk Utility and run First Aid on all Sierra volumes available. If you get an error indication repeat until no error is reported. Then try rebooting normally into Sierra.
If you still can't boot into Sierra reboot into the Recovery volume on Sierra and reinstall the system. That will reinstall the system without overwriting the user data.
Just an update on this one, I took the iMac into an Apple Genius Bar today and they managed to boot it up by holding down alt and then doing a few hardware tests, they showed that the hard drive was fine and they managed to reinstall the Sierra OS, it took a while but it's back up and running again now. Hopefully this might help someone else.
Do you have 2 partitions or is the Sierra install on an external drive?
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Hi, Yes both are on partitions on the 1Tb internal HD. Thanks
Hi thanks for that, I just tried booting into recovery mode but it's exactly the same, it starts up, it chimes, the progress bar starts and then it shuts down.
2010 iMac chimes but switches off after switching start up disc.
