You'll need to wait for a few minutes while the reboot completes. If you are near your modem, you'll see the lights go off, and then start lighting up again as it boots up. Once the INTERNET light turns green, you know you're back online.
Another way to know when the reboot is done is to watch your phone’s WiFi connection. You will lose your WiFi connection when the modem shuts down. Then, once it boots back up and starts sending out the wireless signal again, your phone will either auto-connect OR you'll be able to see your WiFi network show up in the list of available networks.