issue was:
net share from Windows 10 to windows XP failed to work, out of the blue





two ways to solve this:


via gui:
control panel - add remove programs - add remove windows features 
(note that win server puts the add remove windows features in that server gui page)

enable this and the two children
SMB 1.0/CIFS File Sharing Support
 - SMB 1.0/CIFS Client
 - SMB 1.0/CIFS Server


via cmd line:

Set-SmbServerConfiguration -EnableSMB1Protocol $false


of interest to note is that WinXP suddenly stopped being able to see the share
no immediate / visible error message that indicated the root of the problem

based on my knowledge and experience, I was able to determine the SMB1.0 resolution.