irfan sanam,

Hope below steps will help you to fix the fault

Step 01

Open System Settings --> Wireless & Networks -- >> WiFi connection, touch on the Wi-Fi network that you’re unable to connect and select option “Forget.” after that try to reconnect

Step 02

Open System Settings --> Wireless & Networks --> WiFi Connection, press Menu button and select Advanced----> select WiFi Frequency

Check with Auto mode and try to connect with 2.4Ghz band.

Step 03

Backup all data and Reset the phone. after resetting phone try to connect with WiFi