To my knowledge it's 100 metres as the max length, not 100 feet. I've got a 125 foot cable running to my computer, works fine, can pull the full 25mbit (3MB/s) connection through it without issue.
As for the Xbox, I think there are lights near the ethernet port, I'd have to check when I get home to be 100% sure though. I'd say check physical things first. See if there's a nasty kink in there that may have damaged a wire or two, check to make sure all the wires are in the RJ45 connector all the way, stuff like that. It's quick and could potentially save you a lot of headache.
One last thing I can think of is to try the cable both ways, see if the router lights up with each connector.
Just some thoughts popping into my head, it could be so many other things.