Power your router on and off, usually does the trick. If not follow these steps.
From Apple support article: Troubleshooting Wifi on IOS devices
Hold the On/Off button until "slide to power off" appears. Slide to power off your device. When it is off, press the On/Off button to turn it back on.
If you are using a public or commercial network that requires a login or subscription, see this article for more information.
Verify that you are selecting the correct network if you are in a location with multiple Wi-Fi networks.
Verify that the Wi-Fi network settings are correct. You can view and configure Wi-Fi settings using the following steps:
- Tap Settings > Wi-Fi.
- Locate the Wi-Fi network you are currently using and tap .
If you've verified that your settings are correct and the issue persists, try the following:
- Tap "Forget this Network" to reset settings for this Wi-Fi network, including the password. Then try connecting to the network again.
Note: This can be useful if you identify a network issue and want to prevent the device from automatically using this network in the future.
- Reset network settings by tapping Settings > General > Reset > Reset Network Settings. Note: This will reset all network settings, including passwords, VPN, and APN settings.
Note: If the Wi-Fi network uses MAC address filtering, you will need to add the MAC address to the access list for the Wi-Fi router. The MAC address for your device is listed as the Wi-Fi address in Settings > General > About.