imac shuts off during startup

At which part of the startup sequence does this occur?

• When initially powering on?
• At the Apple logo?
• At the loading bar?
• At the login screen?
• Etc?

Can you get this Mac to startup if you boot up in Safe Mode?

Ref: How to use safe mode on your Mac - Apple Support

What have you tried, so far yourself, to resolve this so that we don't try offering potential solutions that may not have already worked for you?

That sounds like a power or hardware issue.

see > If your Mac doesn't turn on - Apple Support

Check the power connections on the back of the iMac and the wall outlet.