Reboot ipad by pressing and holding both the home and sleep/wake buttons at the same time until the apple logo appears on the screen, then let go. Then try to connect to network.
Go to settings, turn wifi off. Toggle airplane mode on/off. Make sure bluetooth is off. Turn on wif (airplane mode off). Try to connect.
Go to settings/genreral/reset/reset network settings. Then try to connect to network.
If its your home router, then reboot router by unplugging for 5-10min. (do this while you're resetting network settings).
If problem persists, then check for firmware updates on your router.(you check router manufacturer suppot website for downloads and instructions).
Wifi Settings greyed out or dim
Recommended wifi settings
If you upgraded to iOS 6 over wifi try
a restore after backing up
I have had problems with an over the air upgrade solved
by a restore and downloading via iTunes
also see here
restore using a backup near end of Doc