follow the steps in this article
If you have done that you may want to consider contacting your ISP provider or possibly even reconfigure your wifi router
Is the issue everywhere or in one place?
If you feel that the iPad is the issue than you may try using it in other places to determine if it happens everywhere or if the issue is happening in one place. If the issue continues you may consider a restore of the device. Here are the instructions http://support.apple.com/kb/HT4137 you will need to set it up as new to rule out a possible software issue. If you have done that than consider contacting AppleCare.
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
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