Have a NET SHARE W$=W:\ on Windows 10
Have a NET USE w: \\win10pc\w$ on Windows 7
Used to be able to connect and save my backups there etc.
This morning, I get status: Unavailable in Windows 7 as reported by NET USE
same for all my other shares between the two.

Both PC's on same Ethernet LAN and in same Homegroup.

I can see \\win10pc\users from Windows 7 but cannot see the drive shares.