Well duh. When I rolled back to Windows 8.1 I didn't check the Device Manager. Sure enough after disabling and re-enabling the Ethernet card it's working. So, I guess if anyone finds this - here's how to fix your Ethernet if the W10 update knocked it out the last couple of days.
So my Windows 10 upgrade has been working really well but after the last update (may be coincidental) the Ethernet port is no longer working. If I go to Device Manager there aren't any warnings. I did the W10 check for driver update but there was nothing. I swapped the cables around with my Smart TV and it is still able to access the internet, so it isn't the cable or the switch. I booted up a Live Ubuntu disc and it was able to access the Internet over Ethernet so I'm assuming it isn't a hardware problem. I rolled back to Windows 8.1 and it still isn't working, so at this stage I'm assuming that the update knocked out whatever Ethernet driver the X51 is using. I don't have any third party antivirus installed, just whatever comes with Windows 8.1/10. Anyone else getting this?  
X51 R2 Ethernet not working after last W10 update