Thanks all! Problem solved.
I had been running Cura 4.2.1 on my Mac with Catalina. Downloaded and ran the .dmg. When launching the app, I get this message: “Ultimaker Cura.app” can’t be opened because Apple cannot check it for malicious software. This software needs to be updated. Contact the developer for more information. Chrome downloaded this file today at 2:49 PM from ultimaker.com. 4.5 overwrote my 4.2.1 app and now I can't run any Cura.