Same problem. But I have lost not only ssh but ping too. This occurs after the expiration of the lease time of IP address. If I increase the lease time of IP addresses on my zyxel keenetic router, then increasing the time availability of the device on the network. But maximum lease time is 259200... This problem only ubuntu, but on debian it's all ok.
PS. Forgot to add. This problem occurs after you reboot the system first time. Before initial configuration - ping is ok and ssh available at any time.
