The problem was that devices on the network were not visible in Windows Explorer's Network folder. The devices were accessible using Remote Desktop Connection, and in many cases, the devices could be contacted within Windows Explorer by simply typing their network address into the address bar. But eventually even that failed.
I'm not sure there's any point listing everything I tried as suggested by a large number of posts here and elsewhere, but if someone suggests I list what I did, then I will provide that list.
My point in making this post is to explain what solved my problem and to ask why this solved my problem.
The problem was remedied by turning off all the hubs and routers in the system and after a short period, re-powering them up. After that, the network devices were all restored in the Windows Explorer Network folder.
What is being fixed when this happens? I'd like to understand the underlying issue. Are there other ways to remedy this type of problem without powering down the entire network's connectivity components?