Stealth Mode can silently auto-enable in a home LAN
This is a public service announcement since I solved the problem.
Stealth Mode can silently auto-enable on your home network if you change a Windows PC from ethernet to wifi.
We had a Windows machine within our home network that had to be physically moved. As a result we had to change from an ethernet connection to wifi. On ethernet the machine had always been set to private network and was easily accessed. After the move to wifi I confirmed that it was still set to private network and could still be accessed.
About a week later it could no longer be accessed and would not even respond to pings.
I stared at the network settings and it still said the machine was on a private network. All network settings appeared unchanged. It should have been working.
Among many things I tried, I disabled the Bitdefender firewall, and suddenly pings worked again. That caused me to check and, to my surprise, Stealth Mode was on for wifi, but off for ethernet.
So I disabled stealth mode, turned the firewall back on, and all was well. I can only assume that the machine thought it was in McDonalds on a public wifi, even though the Windows network type was set to private.