Wired NIC doesn't work but wireless connection *does* work

Have you tried uninstalling the NIC, rebooting, and letting the machine find it and automatically reinstall the NIC?

Always include version of Windows in your posts. :Idea:

Did you try reseating the Ethernet cable in its connectors at both ends? 

Did you look at the back of the tower to see the NIC status lights: The two lights next to the connector indicate the connectivity status and network activity.

Did you reboot your router/modem?

Did you try reinstalling the Qualcomm Atheros ethernet driver from Dell's support page? 

Reboot and immediately press F2 to open BIOS setup. Look for the ethernet entry and make sure it's Enabled. Save the change before exiting setup, but don't change anything else. Then reboot.

When was last time you replaced the motherboard battery?  CR2032 3-volt battery, ~$2.00.

You might try resetting BIOS:

1. Power on and immediately press F2 to open BIOS setup
2. Copy all current settings to be safe
3. Power off and unplug
4. Press/hold power button for ~15 sec
5. Open case and remove motherboard battery - check service manual for instructions.
6. (While you're in there, use some canned air to chase out all the dust bunnies)
7. Reinstall (fresh) battery and close up
8. Connect mouse, monitor, keyboard, and NIC cable
9. Reboot

Does NIC work now?

And there's always a possibility the onboard NIC failed... 

Thanks to both of you for chiming in. On a Saturday, too.. I usually resign myself to no attention until the following Monday.

Response to Blindmelonball:

Have you tried uninstalling the NIC, rebooting, and letting the machine find it and automatically reinstall the NIC?

Not possible. This is the onboard NIC.

Response to RoHe:

Always include version of Windows in your posts.

Whoops. My mistake. :) It's Windows 10 Pro (64) Version 1803 / OS Build 17134.345

Did you try reseating the Ethernet cable in its connectors at both ends?

Oh Yes. I have tried three different cables and have made use of all the ports to ensure there isn't some dead port there.

Did you look at the back of the tower to see the NIC status lights: The two lights next to the connector indicate the connectivity status and network activity.

I am seeing a persistent, blinking amber LED. According to what I'm looking up, that means I'm connected to a 100Mbps network, right? It's possible that my source for this is out of date.

Did you reboot your router/modem?

Yes. It was the first thing that I did. I powered down, unplugged, and then waited a good ten minutes before reinitializing the network. All other devices in our home can maintain a wired connected with this router/modem.

Did you try reinstalling the Qualcomm Atheros ethernet driver from Dell's support page?

Yes. I have downloaded and installed the Qualcomm Atheros AR8171/8175 PCI-E Gigabit Ethernet Controller Driver (Release Date 22 Jul 2016). I then rebooted with no change in condition.

Reboot and immediately press F2 to open BIOS setup. Look for the ethernet entry and make sure it's Enabled. Save the change before exiting setup, but don't change anything else. Then reboot. 

When was last time you replaced the motherboard battery?  CR2032 3-volt battery, ~$2.00.

This computer is a year old refurb. Wild guess it's not the Mobo battery because I'm not experiencing any CMOS related problems or any other symptoms.

You might try resetting BIOS:

1. Power on and immediately press F2 to open BIOS setup
   ….
2. Reboot

Does NIC work now?

I don’t have a spare Mobo battery, though I will purchase one or two to have on hand once I make another parts-order.

And there's always a possibility the onboard NIC failed...

I'm beginning to think that this is what's happening. Since NIC's are relatively inexpensive, I'll purchase one. I just wouldn't have expected an onboard NIC to die this early, but Manufacturing Defect RNG can be cruel. :)

Thanks for the help!

Check your driver version in Device Manager. In my XPS 8910, the driver version is 2.1.0.17 and the driver date is 9/29/2015 for the Qualcomm Atheros AR8171/8175 PCI-E Gigabit Ethernet Controller. Dell driver release dates are not a reliable indicator, click on "View full driver details" to see the driver version under "Important information". In this case for Windows 10, the version is 2.1.0.17, Windows 8.1 is 2.1.0.21, and Windows 7 is 2.1.0.25 all for the driver released on 22 Jul 2016..