MacOS Catalina upgrade

I am upgrading my iMac from High Sierra to Catalina. The software download finished at 9:30 am yesterday and I started the upgrade 33 hours ago and it is still going on. I had a white screen for 12 hours and the Catalina log in screen since then with the prompt “Setting Up Your Mac”. It has never taken anywhere near this long to upgrade the MacOS before. What is the problem?