One of the most common reasons due to which you might be not able to use wired internet on your Mac is incorrect IP intake by the system. If you are using an ethernet cable to use the internet then do the following steps :-
>>Connect the LAN/ethernet cable into the LAN port on your computer.
>>Go to the apple menu, then System Preferences, and click Network.
>>You will get the network connection list, now choose Ethernet from it, make sure it is not in inactive state.
>>Now go to the configuration menu, here you will have to put in the configuration details as provided by your ISP, you can do it in two ways, first you can select to obtain IP automatically using DHCP or Using BootP option. Second you will have to do it manually, enter the IP address, subnet mask, and router address you received from your service provider. (If it is already on automatic the put in the details manually and vice versa)
>> Now put in the IP address of your ISP’s DNS server in the DNS Servers field.
>>Now click on apply. And you are done.
Hope this helps !!!