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
iOS: Recommended settings for Wi-Fi routers and access points http://support.apple.com/kb/HT4199
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
If none of the above suggestions work, look at this link.
iPad Wi-Fi Problems: Comprehensive List of Fixes
Fix iPad Wifi Connection and Signal Issues http://www.youtube.com/watch?v=uwWtIG5jUxE
Assuming the problem is a carrier problem (you dont say) open iTunes on device, music tab, scroll to bottom of screen and click on sign out. (email will be displayed in bottom left tab if already signed in).
Click tab again and tap on View my AppleId - sign in.
If still unable to sign in, go to Settings\General\Reset Reset Location Warnings and reset.
Check you can sign into Safari and the App store. If you cannot sign in to either go to Reset again and Rest Network Settings., if problem still persists, use Reset all Settings. Sign out back into iTunes as given before afterwards
If you connect to a PC, sync to iTunes when finished doing any resets. Confirm you can sign in to your account from iTunes Home page using the QuickLinks section. When in your account, ensure that your devices are listed in the Manage Devices under the iCloud section.
At the bottom of this page is also the iTunes Store links section. Make sure under the Manage\Change Country option that your iTunes country store is correct. (Displays in the Main Account page under Apple ID Summary\Country/region).
Very ocassionally you can get a cached warning problem which blocks connection attempts. With your device connected by cable to your PC right click its icon in iTunes and select Reset Warnings. Do the same under the main menu item Edit\Preferences Advanced tab, click to reset Dialogue Warnings and reset Reset Cache. resync your devices when finished.