SWMBO's iMac has stopped talking to the LAN.

No matter which router port I plug her Ethernet cable into, that port's activity light on the router stays off, and she has no LAN connection. My computer talks to the LAN when connected to any one of the four router ports, so the problem is probably not the router.

I temporarily connected the iMac to the router with a known-good Ethernet cable (very awkward cable routing because the temporary cable was too short for usable ergonomics) and the non-connect problem remained unchanged. The last time she used the computer (two nights ago) she had connectivity.

My knowledge of LAN connectivity extends about as far as plugging the Ethernet cables into the receptacles right side up. What should I look for to try and solve this problem?

tanstaafl.