Sometimes it can be as simple as rebooting the router. Sometimes just restarting the iPad will help as well. Try both and then if you still cannot connect - start going through the suggestions in these two articles.
Look at iOS Troubleshooting Wi-Fi networks and connections http://support.apple.com/kb/TS1398
Additional things to try.
Turn Off your iPad. Then turn Off the wireless router & then back On. Now boot your iPad. Hopefully it will see the WiFi.
On your iPad go to Settings > General > Reset > Reset network settings and see if that enables you to connect.
If none of the above suggestions work, look at this link.
iPad Wi-Fi Problems: Comprehensive List of Fixes