No internet connection with ethernet cord plugged in

Go to Network preferences within System Preferences and make sure there is a Ethernet interface listed on the left. If there isn't one for some reason, add it, then click the Apply button in the bottom right. If it is there, highlight it, and see if it is set to configure IPv4 using DHCP. Change it to that option if it isn't already and apply the configuration again. Then see if there is any info in the fields for IP address, subnet, router, DNS. You should generally have an IP beginning with 192.168.x.x, 172.16.x.x, or maybe 10.0.x.x if you have an Apple router. If you have an address beginning with 169.254 or a self-assigned IP, the other fields will generally be blank and you may want to check the Ethernet port, cable, or the settings on the router itself to see if it is in bridge mode. If all else fails, restart the router or replace it with another model.