If you have a passcode setup on your iPad and enter the wrong passcode too many times, the iPad will become disabled, locking you out of it. After a few missed attempts, the iPad will disable itself temporarily, asking you to try again after a minute. But if you continue to type in the wrong passcode, the iPad can disable itself permanently.
But I my iPad is disabled and I didn't type in a wrong passcode.
If your iPad is disabled, someone typed in the wrong passcode enough to disable it. Even an iPad that did not have restrictions on can turn up disabled if someone enables the passcode and then types in a wrong passcode multiple times.
Most of the time, this is just temporary. For example, after locking you out for a minute, if you type in the wrong passcode, the iPad will lock you out for five minutes. This will duration will be stated at the top of the screen.
If your iPad has become disabled permanently, your only choice will be to reset the iPad in iTunes and restore from a backup. This may seem like punishment, but it is actually for your own protection. If someone stole your iPad and tried to unlock it, the iPad would become permanently disabled, thus keeping the person from getting access to your iPad's data.
Remember to restore the iPad from the same computer you used to sync. If you are using iOS 5.0 and have iCloud set up to backup your iPad, you can also restore from the cloud. Even without restoring from a backup, you can go to the app store and install any app you have previously purchased for free.