iMac shuts down all the time

If you have an earlier model iMac you might want to reset the NVRAM and the SMC

This steps only apply to intel based Macs

To reset your NVRAM

1. Shut down your Mac.
2. Turn on your Mac, then immediately press and hold these four keys together: Option, Command, P, R.
3. Release the keys after about 20 seconds, during which your Mac might appear to restart. For example, you might hear a startup sound more than once, or see the Apple logo more than once.
4. When your Mac finishes starting up, you might need to adjust any system settings that were reset.

Reset the SMC

Again this steps only apply to intel based Macs without the Apple T2 chip

If your Mac isn't a laptop computer, follow these steps:

1. Shut down your Mac, then unplug the power cord.
2. Wait 15 seconds, then plug the power cord back in.
3. Wait 5 seconds, then press the power button to turn on your Mac.

If you have an Intel Mac with the Apple T2 Security chip follow these instructions to reset the SMC

1. Shut down your Mac, then unplug the power cord.
2. Wait 15 seconds, then plug the power cord back in.
3. Wait 5 seconds, then press the power button to turn on your Mac.

If you still need help  contact Apple Support. 

Download and run Etrecheck. Be sure to give it Full Disk access before running.

Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Additional Text button to paste the report in your reply.

Then we can evaluate the report to see if we can determine the cause of the problem.

Do you have Win Key & alt key?

If so...

Win key = Mac CMD key

alt key = Mac Option key.