First, try another cable. It's amazing what can happen to electronic components that are tucked away out of sight...oxidation, dust motes...
I've got an ethernet connector on an AP that I've redone 3 times...and it's sitting out of the way where it doesn't get touched, yet every now and then the connection just gives up and I change the connector.
Steps for diagnosing lack of ethernet conectivity;
1. Plug the cable into a different port on your access point (router, hub, modem, or switch)
2. Try a different cable.
3. Plug cable into a different computer
4. Reinstall Network drivers.
5. Install a different network card (if you're skill level allows it).