With any luck you have a bad ethernet cable - try another one. Also try another ethernet port on your “modem”. (That is assuming it is a modem/ router combination.)
If that doesn’t work (and it probably won’t), try powering the device off for a few minutes and see it it recovers when you power it back on.
You could also try a short reset, but I’m not sure that would do anything to fix this problem. (It will reset admin password, device name, static IP address, and other similar customizations.) The last resort would be to reset to factory settings, but I think that wipes out your data. Do you have a backup?
But speaking of customization, did you do anything shortly before your MBL stopped working? Setting or changing a static IP address? Changing routers? (The new one might use a different IP subnet.)