I've lost wired connection in Debian.
Yesterday it worked perfectly. The network-manager says 'cable unplugged', which is false.
How to find the cause, and solve that?
Quickest solution - get a laptop or any other device with a network port and use the shortest ethernet cable you can find and connect. See if it still says cable unplugged.
If it doesn't say cable unplugged, the issue could be with a break in cable.
If it still says cable unplugged, either the device doesn't support direct connections without a cross over cable (but, unlikely now-a-days), or the port on your machine may have gone... In this case, I would quickly do a reboot in the hopes it will fix itself, or try a live cd to see if you can get it working from another distribution/install.
If you can get it working elsewhere, you know it is a local problem (driver or similar) where as if it doesn't work elsewhere, most likely the socket is dead.