I'm a random reader, sorry, but thought I might be able to help.
If you are using OS X, going into the network utility and try to send a "ping" (basically the computer sees if it can connect to an IP address by sending ten small packets of information to be repeated back to it) to the router (look on the bottom of the router for the IP address). If you get a ping back and everything is good, then it's just settings that are screwed up.
If the ping doesn't return, then it's most likely hardware issues. I agree with ckd in that you should check different ports on the router, but also try swapping cables. It could just be a frayed cable (note: it can be frayed internally without showing signs on the outside).
no subject
If you are using OS X, going into the network utility and try to send a "ping" (basically the computer sees if it can connect to an IP address by sending ten small packets of information to be repeated back to it) to the router (look on the bottom of the router for the IP address). If you get a ping back and everything is good, then it's just settings that are screwed up.
If the ping doesn't return, then it's most likely hardware issues. I agree with ckd in that you should check different ports on the router, but also try swapping cables. It could just be a frayed cable (note: it can be frayed internally without showing signs on the outside).