There are a number of troubleshooting steps you can follow to get connected to the internet again.
1) Make sure that your router is on and you're within range
2) Make sure that Wi-Fi is on and you can see your network. Go to Settings > Wi-Fi and make sure that Wi-Fi is on. Tap the name of your Wi-Fi network to join. A blue checkmark beside a network name means that you're connected.
3) If you still can’t connect to your network or get online, make sure that your router is connected to the modem and turned on.
4) Restart both your device and your device, router, and modem.
5) Reset your Network Settings. Tap Settings > General > Reset > Reset Network Settings. This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you’ve used before.