Xbox Error 73
The Xbox Error 73 is a hardware error related to the ethernet port. Like many other Xbox problems, it could be caused by faulty solder connections and you might be able to fix it yourself.
Make sure your network cable is good by switching it with another cable you know is good. Sometimes you can clean dirty contacts just by unplugging the cable & plugging it back in.
If that doesn’t work, check the rest of the cabling to your internet connection & see if you can still get online from a computer. Another computer may be streaming a movie or downloading a huge file making the Xbox think it has a connection problem.
Xbox Error 73 Repair Video
The methods shown in this video may not fix the e73 error and may damage your Xbox. If you decide to try it, you're doing so at your own risk! If your Xbox is still under warranty you should seriously consider sending it in for repair.