Quite a common problem. If you did all that Shiham recommended and still no joy , here are solutions you might try.
1. If using a pc/laptop connected to a wifi router and you are sure your data plan is still valid (ie you have enough data) then try the following : a) Switch off the router (at te plug if necessary) , wait for 60 seconds and power up again and see if it works. If not then:
b) Even if your device recognises your router but still no I/N , click on your I/N icon (bottom right on the service tray) to open the WiFi pane. There you'll see your wifi router listed (normally the top one with the strongest signal icon). In this block look for a button that says “Forget this Network “ Click it.
Now , power off your device , wait a few seconds and power up and wait until device has finished booting up.
Now again click on the I/N icon in the service tray. You will see your router listed. Click the CONNECT button. It will then ask for the router ID/Password or Key. (This is the long alpha - numeric code found on the label underneath the router).
Type this code in EXACTLY as it appears on the label (sometimes there is a button that says Show Password or a small EYE-icon) click on it and it will show what you type in so you can check it's accuracy - instead of just showing black dots.
Then click OK/Next or Connect.
This ought to clear the problem. PS. Don't forget to tick the Automatically Connect box under the settings or preferences button.
# If this problem is on your mobile phone , obviously make sure you're device is on wifi. The above procedure ought to clear the problem on this device as well.
# If you are able to connect to the I/N via your router (an excellent test) but not via mobile data , the most likely cause is you've run out of data.
# I have solved the above prob many times for aquauntences of mine but if still no luck and you've tried the Google thing to solve it without success , then try your ISP service provider who will help reset your router , or your mobile data company with whom you have a voice/data contract with.