Assuming that your wireless devices are connecting OK, there are really no settings to adjust other than turning off wireless on the Mac.
Try another Ethernet cable.
Restart the AirPort router
Try another LAN <--> port on the AirPort router
Restart the Mac
If you are still having difficulty, try connecting another computer to the AirPort using an Ethernet connection. If that works, that tells you that the AirPort router is working correctly......and you have have a problem with the Ethernet port on the Mac.
If the other computer cannot connect to the AirPort using Ethernet, then the Ethernet switch on the AirPort may be acting up. That will require a trip to the Apple Store.....if you have one in your area.....or to an Apple Servicer to let them have a look at the AirPort router.