lost onboard network connection on stretch

[Gary Dale <garydale@torfree.net>]

After an upgrade & reboot last week, I lost my onboard network 
connection on my Stretch/AMD64 system.

I can still see the device using lspci:
05:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. 
RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller (rev 06)

but it doesn't come up. I've attached an old Linksys USB network adaptor 
and it gets recognized and gets a DHCP address but not the onboard NIC.

Any ideas on what's gone wrong and how to fix it?