What is the make and model number of the motherboard (or PC)?
What OS is installed, Win 7, 8.1, 10?
Have you checked in Device Manager on the status of the ethernet Network Adapter, whether it is disabled, needs a driver update or if there is some other problem?
To get to Device Manager in Win 10, right click on the Windows Start button on the left side of the Taskbar and click on the Device Manager link in the options box that appears.
In Device Manager scroll down to find the Network Adapter entry and click on the arrowhead next to it to expand the list.
If there is a red cross next to the ethernet network adapter (I don’t know what its make and model number is in your computer as you provided no information), right click on the entry with the red cross and select enable and then test the ethernet connection.
If there is a yellow exclamation mark next to the entry, right click on the entry with the yellow exclamation mark and select update driver and then follow the prompts. If you have no WiFi internet access you’ll have to get online somehow else to the ethernet adapter manufacturer’s website and find drivers for the particular adapter appropriate to the OS installed, which is used in the computer.
If there is no red cross or yellow exclamation mark next to the ethernet entry right click on the entry and select Properties and check the status of the device in the General Tab.
Here’s an image to show what I mean. It is not the same adapter as in your computer. It is only to show you where to look.
(click on image to enlarge for better viewing)