First step i would try if you haven't already is pressing and holding the reset button on the back.
Then cycle the system by unplugging th modem and router. Plug the modem in and let all the lights come back on like they should (other than the internet light which won't until the router is back). Then plug the router up and let it fully start up.
Now you log into the router and check all the wireless settings including any wireless security you set up. Check the wireless devices and make sure they match.