Method 1 - Change WiFi Setting
Step1: Navigate to Settings > Wifi > Long select on your Wifi Network to see Modify Network Option
Step2: Select Modify Network option
Step3: Enter password
Step4: Check Show Advance option
Step5: Scroll down to view advance options which must be available by now.
Step6: Change IP Settings from DHCP to Static
IP Address : Most probably your Gateway address will be 192.168.1.1 change your IP Address my changing last digit in this address use any value preferably between 2 to 20.
Change DNS1 to 188.8.131.52
Change DNS2 to 184.108.40.206
Step7: Save these settings.
Step8: Login into your wifi network if required, you device should be able to connect to internet by now.
Method 2: Reset Router and Wifi Network
Step1: Navigate to Settings > Wifi > Long select on your Wifi Network to see Forget Network Option
Step2: Select Forget Network
Step3: Switch off your router and wait for 10-15 seconds before restarting it.
Step4: Restart your Wifi router
Step5: Reboot your phone
Step6: Search for your Wifi Network in available Network lists.
Step7: Connect to your network and check if device is connected to internet or not.
Step8: If you are connected to internet by now then you are done, else perform Step 2 to Step 8 from Method 1.
Update your phone and Router date and time to match internet provider date and time to check if all three are in Sync.
If none of above method works than download any Wifi issue Fixing application such as WiFi Fixer. This application will analyze and resolve issues if your device is connected to wifi but not able to connect to interne