Thank you for your concern and assistance.
- All the trouble-shooters fail to find any problem.
- Your suggestion to run DISM was interesting.
However, I ran the tool as indicated. It did not report anything.
The problem File and Printer sharing always reverts to OFFL
-> change sharing options
-> turn on file and printer sharing
-> save changes
return to 1, the sharing options are off again.
- Other people appear to have met the problem:
(see http://www.tomshardware.co.uk/forum/id-3372390/windows-discoverable-network.html )
After this action, the media folder of a computer on the network becomes visible, but not the files nor the printers.
After reboot, the entire problem remains : my computer is not visible on the network, and no other computers visible on the network from my computer.
- I am trying to go back to a Windows restore point.
This fails however, apparently a running process prevents the restore from completing.
However, I have no anti-virus running and no other background process that I can see.
Thank you for your support.