Most router issues are due to a problem with your Internet connection. Re-booting your router, or turning it off and on, solves most connection issues:
Unplug your router.
Wait 1 minute.
Plug your router back in.
Attempt to connect to the Internet.
If rebooting your router didn't solve the issue or if you don't have a connection issue with your router, review specific troubleshooting tips for your router.