Different Comcast tech came out. Customer has an old Netgear Comcast modem/router. Apparently to reset one of those, you have to hold the tiny reset pin for over 2 minutes (did not know this). I wondered why it didn't seem to reset when I held it for about a minute.
Anyway, that got it working again. So some kind of memory corruption. I did tell the customer if it happens again, it is probably the sign of a deeper issue with that modem/router and they should insist it be replaced.

As he left, he held out his hand and I shook it. I guess old habits are hard to break, especially when we are not thinking. I did immediately go to the bathroom and wash my hands. Got to think: How many people this guy sees a day and if he is shaking all of their hands....
As of this week, we have confirmed cases all over the area. So many, many people are infected here now.