Xbox e73 Error
The e73 error 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 known good cable. Sometimes just unplugging it & plugging it back in cleans dirty contacts enough to solve the problem.
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. It may be as simple as an internet connection problem.
Xbox e73 Error 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.