due to some security issues (Wannacry) Windows administrators are going to disable the unsecure smb1 protocol.
mount.cifs is using smb1 by default. So when you tried to connect to a Windows machine where smb1 is disabled you get an error: mount error(112): Host is down.
root@devdeb ~ # mount -t cifs -o user=yourWindowsUser,workgroup=yourDomain //yourWindowsMachine/share\$ /mnt/yourWindowsMachine mount error(112): Host is down
To establish a connection nevertheless you have to force the use of smb2
root@devdeb ~ # mount -t cifs -o user=yourWindowsUser,workgroup=yourDomain,vers=2.0 //yourWindowsMachine/share\$ /mnt/yourWindowsMachine