Is the connection between the PC and the router just a single direct Ethernet cable?
Or is there some interim component; e.g., an Ethernet wall jack, an Ethernet connector making two short cables into a longer cable?
Be sure the male Ethernet plugs on both ends are firmly in place and squarely connected to the wire.
Try a third known working Ethernet cable. Ensure that each end is firmly and snuggly connected to its' respective receiving port.
PCI_x network adapter? Open your pc up and ensure that the network adapter is fully seated in place. Do the same if you are using some form of USB to Ethernet adapter. Overall, make sure that whatever network adapter is being used is tightly in place.
Look for debris inside the Ethernet ports on both ends. Anything at all (could be some piece of paper, a dust bunny, a dead bug) that may be interfering with connectivity.
Try another router LAN port if possible. Temporarily disconnect a working computer and plug in your computer.
Check the router's logs (if available and enabled) for errors.
Check your computer's Event Viewer logs as well.
The goal is to narrow down the problem source: e.g., router, cable, wall jack, network adapter,etc...