iMac locked up during upgrade to Catalina 10.15

I have an iMac (2012 or later). I downloaded Catalina 10.15 and followed the prompts to install the upgrade. It got to the point of about 50% complete and has hung up with the prompt changing from %complete with estimated time remaining to now just sitting with the prompt “estimating time remaining”. It sat for over 24 hrs with no change. I did a hard shutdown and restart and it immediately goes back to this state/prompt with the progress bar back at about 50%. I can not get it out of this state. Any help would be appreciated. Thanks