Usually it's because you are not making internet connection via wifi.
Look at iOS Troubleshooting Wi-Fi networks and connections http://support.apple.com/kb/TS1398
iPad: Issues connecting to Wi-Fi networks
http://support.apple.com/kb/ts3304
Additional things to try.
Try this first. Turn Off your iPad. Then turn Off (disconnect power cord) the wireless router & then back On. Now boot your iPad. Hopefully it will see the WiFi.
Change the channel on your wireless router. Instructions at http://macintoshhowto.com/advanced/how-to-get-a-good-range-on-your-wireless-netw ork.html
How to Quickly Fix iPad 3 Wi-Fi Reception Problems
http://osxdaily.com/2012/03/21/fix-new-ipad-3-wi-fi-reception-problems/
If none of the above suggestions work, look at this link.
iPad Wi-Fi Problems: Comprehensive List of Fixes
http://appletoolbox.com/2010/04/ipad-wi-fi-problems-comprehensive-list-of-fixes/
Fix iPad Wifi Connection and Signal Issues
http://www.youtube.com/watch?v=uwWtIG5jUxE
Cheers, Tom 😉