A:
Well your question is lacking the details as to what kind of router your using, and what you plugged it into.
1. With the router plugged in and the power on hold down the reset button for 30 seconds. ( the lights should start fast blinking) the router will then reset ( can take up to 5-10 minutes). When that is done you need to reconfigure it from your computer.
2. I the above does not work, from your computer open system preferences and remove/forget the router from your network. Power the router off and back on. once that is done repeat set one.