My iMac has issues since last iOS update

My iMac doesn’t simply start and login like it did before the update. The login screen freezes, then reverts to the black update screen, then hesitates when I login the second time. There’s box about opening previously opened programs with a countdown timer. Then the desktop screen freezes. (Sigh) I have to wrestle with sign in 4 hours before my Zoom class and open the app so the computer doesn’t go to sleep and require 20 minutes for login.