Have lost my ethernet connection

Computer was connected to Internet via ethernet > router-modem this morning. Left the house for an hour, when I returned I had no ethernet connection. I switched to Wi-Fi and so can connect to the Internet that way, but why have I lost my ethernet connection?

I have tried all manner of restarts: router-modem and computer off, then restarted in various sequences, still nothing.

The ethernet connection light on the back of the router-modem comes on when the ethernet cable is plugged in, so it appears I have power there. But not connection

Any suggestions appreciated.