Puzzle about networking. Any suggestions?

All of a sudden and for no apparent reason my ethernet connection isn't working. Its not the router, other machines on the router work fine. Its not the local powerline device, I have changed that out. The machine also works fine with a mobile data dongle plugged in to a usb port. So I think it must be something about the configuration, though why it should suddenly have changed is a mystery to me.

I do ip link, and this shows as number two:

2: enp3s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000 link/ether 08:60:6e:e7:d2:be brd ff:ff:ff:ff:ff:ff

So then I do ifup enp3s0, and get this:

~$ sudo ifup enp3s0 ifup: unknown interface enp3s0

When I use the gnome network control tool it fails to bring it up.

ip link set appears to work, at least no error message.

ifconfig gives this:

enp3s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500 inet6 fe80::d2da:494:eadc:4e0b prefixlen 64 scopeid 0x20<link> ether 08:60:6e:e7:d2:be txqueuelen 1000 (Ethernet) RX packets 0 bytes 0 (0.0 B) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 982 bytes 158082 (154.3 KiB) TX errors 1 dropped 0 overruns 0 carrier 0 collisions 0

So why can't I bring it up with ifup?

Rebooting doesn't make any difference. I don't know what else to try. Its probably something very simple that I am just missing....

Peter