Sharing via Workgroup suddenly stopped working; semaphore timeout

echo "Hello World!";

Got a new PC last month; all was fine for the first few weeks until yesterday when Win 10 (Enterprise) decided it couldn't access any shared workgroup folders - and that my other PCs couldn't access its own shared folders either. Now when I try to click a link to a shared folder on another PC, Directory Opus (eventually) gives me a semaphore timeout error. Windows Explorer does nothing. Network folder shows only my router. & When I'm trying to get in here via other PCs I just get the standard "not available" error I get as if it's switched off.

It is the "main" PC of the group, i.e. the one wired into the router. Other PCs (and my Android devices) can access other PCs shared folders.

I have already updated my Realtek drivers. Settings in Network and Sharing centre all OK. I have changed no other settings that should affect this. Internet access all OK.

Edit: have tried System Restore, to two different points. Still no way in or out on the workgroup folders. Homegroup is not an option for me BTW.

Still, Windows 10 is given away for nothing, isn't it.