It’s always good to get the basics out of the way first. There’s no point going troubleshooting if a simple reboot fixes the issue, so try these first even if you don’t think it will fix anything.
1. Reboot your router and try reconnecting to Wi-Fi from your phone.
2. Reboot your phone and try reconnecting to Wi-Fi.
3. Verify Wi-Fi by connecting with another device.
1. Select Wi-Fi from the Quick Settings drop down on the home page.
2. Select Forget and then reconnect.
Try manually setting the frequency.
1. Access the Wi-Fi menu from Settings then select the three dots in the top right.
2. Select Advanced, then Wi-Fi frequency band. It should be set to Automatic.
3. Manually select the frequency used by your router.
4. Try to reconnect to Wi-Fi.
If none of those steps work, our step of last resort is to reset all connection from the phone. This will also reset your cell connection and Bluetooth settings so use at your own risk.
1. Navigate to Settings then Backup and reset.
2. Select Network settings reset and then Reset settings.
3. Once the reset is complete, attempt to reconnect to Wi-Fi.
one of the above steps is sure to work out for you.
Source: Infocity blog