Solved: We managed to limp through the day and get patients seen and out of the office after which I could return my attention to debugging this.
First, my sincere appreciation to many prompt replies and helpful suggestions. I find the user board invaluable for the networking collegiality.
Unfortunately, restarting main computer and clients made no difference.
Client FileExplorer\\<MainComputerLocalNetworkAddress> also was unsuccessful.
The solution may be more useful to others if I restate the problem a bit more clearly.
First some definitions:
"Desktop" means a computer housed in a metal tower, not portable.
"desktop" means Windows OS user window.
Network: Peer to Peer, 1 Main computer and 4 clients, all Windows 10 but a mix of Home (Laptops) and Professional (Desktops)
All files (AmazingCharts and otherwise) stored on Main computer - several thousand, many confidential from staff.
Client staff accounts given access to about 100 shared files on Main computer (timecards, cash drawer ledger, inventory, forms). This is done by going on Main computer FileExplorer navigating to file to be shared and right clicking ShareWith/SpecificPeople and then individually giving each staff user either Read or Read/Write privileges. Established over 9 years of practice file by file as various files developed, I have no list of what has been shared, and it would be very difficult and time consuming to recreate this.
Main Computer automatically upgraded to Windows 10 Pro "Creator" Version 1703
Thereafter, staff user client FileExplorer initially would not display Main computer. Interestingly, physician user client Laptop FileExplorer still could access Main computer shared files, but the shared files are all owned by the physician user on Main computer.
However, client computers staff users still could log on and use AmazingCharts, and client staff user desktop shortcuts still navigated to shared files on Main computer and displayed them correctly. Once a client computer accessed a desktop shortcut, then the client FileExplorer would show the Main computer, but NOT any of the shared files. The only shared files accessible were the few with shortcuts on client staff user desktops, and only when accessed from the shortcut, not from FileExplorer on which they were still not displayed at all.
Main computer FileExplorer, navigate to any shared file, right click, ShareWith/SpecificPeople. Still shows all original permissions intact. None the less, for every user, again enter either Read or Read/Write permissions unchanged and click "Share". Just once, and just for any one random previously shared file, not all previous shared files.
Now, client machine staff user accounts FileExplorer all navigate and display all 100 previously shared files.
I certainly don't understand why this happened or why the above actions restored access, but we are back in business.