Suddenly no sound coming out of iMac

No sound coming out of iMac. Only have internal speakers. When I checked to see if mute

had been inadvertently checked during the latest update, not only was it NOT checked, but

I could not even see the drop down box for internal speakers.

What to do? Any suggestions? Please help. This is a particularly bad time to lose sound given

Zoom conferences, FaceTime, etc. as the only way to connect or do any work for several weeks

or even longer.

Thanks!