You will have to do some basic troubleshooting.
It may be a SOFTWARE GLITCH or a hardware glitch.
1. Defective ethernet cable
2. Ethernet chip (on motherboard) defective.
Sometimes a power surge can wreck things, but we'll likely never know. Basically you have to swap parts or prove components.
1. Hook your laptop to the ETHERNET cable that the PC used. Set it to use that and NOT the wireless network. If that works, your problem is inside your desktop PC.
2. Reinstall the Ethernet driver software (your exact motherboard)
If you've proved the ethernet cable works (thus proving the modem/router as well) thus proving the desktop PC and software reinstall of Ethernet drivers don't help it may be the Ethernet chip is damaged.
At this point, I recommend buying either a PCI/PCIe or wi-fi adapter for about $30. Turn off your LAN in the motherboard BIOS since if it's damaged you don't want that on.
I hope this helps a bit.