Thanks svennilenni. I thought after I posted that I should have done that so … Mac OS Mojave 10.14. Now that I think of it, I also updated the os since the last recording session. Maybe an os problem?
Edit: It turns out that it was the OS causing the problem. The first time I fired up Cubase after installing Mac OS Mojave it asked whether or not it could have access to the Macs’ microphone … I said no because there was no need to access the built-in mic in the Mac. But … it turns out that in this context, “microphone” means “any input”. It was fixed easily be going to System Preferences/Security and Privacy/Privacy/Microphone and checking Cubase. Problem solved.
Thanks again, svennilenni, for reminding me about the os.