Mac screen is black

Please follow all of the steps at > If your Mac starts up to a blank screen - Apple Support

If those do not work, then you need to contact Apple Support and/or make an appointment at your local Apple Authorizes Service Provider.

That does suggest a dead display, or at least a dead backlight.

1. Try holding 'option' on startup to see the boot screen, or Command+R for recovery mode. If that works then you know the display works and the problem is an early system crash/shutdown.
2. Again, try connecting to an external monitor. (If you don't have one you can connect to an HDMI TV using a USB-C to HDMI cable (2017 or newer iMac) or a MiniDisplayport to HDMI cable (2015 and older). If the iMac works with an external display you then have confirmed that you do have a display related hardware problem.

Alternatively, take it to an Apple Store or AASP for diagnostics and/or repair.

Which iMac do you have, and what version of MacOS is installed?

Do you hear a startup chime?

If you connect an external monitor do you see anything on that?