I forgot to mention one thing:
When I disable the LAN connection and then re-enable, I see a “LAN is now connected at 10 mbps” balloon pop-up for 3-5 seconds, then the “a network cable is unplugged” message comes back.
If this was just some harmless device problem, which could be easily fixed, wouldn’t enabling/disabling the connection NEVER produce a “now connected” message?
That’s what has my internet.
Bizarre.
It sounds as though your computer’s network card is working normally and you said you’ve swapped out the ethernet cable. Assuming you’ve tried powering down your modem for two minutes and had no luck, I would start suspecting that it has a hardware failure, as it doesn’t sound like there are any software issues with your PC. Do you have another computer you could try to connect to that modem to see if it will connect? Alternatively, is there any other location (at a neighbor’s house, your office, etc.) where you could try connecting your computer to a network to see if it works there? If not, you may be stuck with having to reach your provider’s tech support.
I would be strongly suspecting the modem at this point. If you rent, your service provider should swap it out for you. If you own it, you may be on the hook to get another one; hopefully you have a local store with a decent return policy so you can bring a new one back if it doesn’t work either.