Help! Cannot Reboot iMac

A message popped up on my iMac (running Catalina) indicating that updates that were installed during the night required that the computer be restarted. While going through the start-up sequence, the screen displayed a "small circle with a slash through it". After waiting about 40 minutes with the symbol still remaining, I shut down the computer using the power button. When I restarted, again, the computer went through the normal start-up sequence. After a couple of minutes, the same symbol was displayed.

I never experienced anything like this before. Could someone please help? Thank you so much.