Hi Ministrel,
Welcome to the Apple Community.
It would help us to help you if we knew which iMac we are discussing.
(Apple icon in top menu bar > About this Mac > will show the year.)
Sometimes a setting or two has been overlooked.
On some current Macs it can be found:
(Apple icon in top menu bar > System Preferences > Security & Privacy > Privacy < Camera < ensure Zoom app is ticked.)
Also in Safari app (if that's the browser you use) in current versions it is:
Safari > Preferences > Websites > Camera > Ask or Allow (not Deny)
Apple Support have an article online:
If your built-in camera isn’t working on your Mac
https://support.apple.com/en-us/HT211130
Hope something here is useful :-)