Mac camera not working

What year model iMac and what macOS version...?

see > Identify your iMac - Apple Support

To troubleshoot, see > If the built-in camera isn’t working on your Mac - Apple Support

Was this iMac recently opened for service, a repair or an upgrade...?

If so, then maybe the camera is disconnected or damaged.

Are you really running a Mac from 2014 or earlier? It could need a reset of the SMC:

Reset the SMC of your Mac - Apple Support

Many built-in ports and accessories of machines older than 4 years no longer speak to the BIOS, and that requires a reset of the SMC to fix, since the BIOS battery or capacitor also known as the PRAM no longer gives any charge.

Go to Apple menu -> About This Mac and verify what OS you are running.

There is a software called Photobooth that can verify if the camera is working too.