2011 MBP Ethernet Port not showing up (Sierra)

My ethernet port has not been working. To the advice of some threads I read on the internet I deleted the Ethernet from my network preferences and rebooted my computer. This unfortunately did not cause the port to reappear and when I try to click the "+" to add a device the Ethernet port is not shown.

How can I get it back? Running Sierra 10.12.6.

 

Thanks for the info. I did check the System report. The Ethernet card was showing in my Network preferences before I deleted it, as was info in the System Report. After delating the Ethernet port from my Network preferences and restarting the computer the Ethernet port no longer showed up in the Network preferences and in the System Report. In the latter all you see is this under the Ethernet card info:

"This computer does not appear to have any PCI Ethernet cards installed."

I had read there was an Apple update that messed people's ethernet cards and caused them not to be recognized. Someone suggested getting an uncorrupted/updated file from another working OSX system and then adding that to the system/library/extensions. I did this but that didn't seem to help. Though my friend isn't very computer savvy and she could not confirm the last time she used her Ethernet port, so perhaps she could have the same software issue.

 

Is there a guide to doing this? I changed out the original drive for an SSD, cloning the other drive. This was awhile ago and had no issue with the ethernet port.