When I turn on my desktop PC, the ethernet port it was plugged into the day before on my Comcast-supplied TG1682G modem/router frequently doesn't respond. I have to plug the Cat-5 cable into each port until I find one that works. Restarting the unit doesn't fix the problem.
Try hard resetting the device to factory defaults by pressing and holding in the recessed reset button on the rear for 30 seconds. This can sometimes clear up some weird behavior issues. You will lose any customized settings, and they will need to be re-configured from scratch.