You must restore it, http://support.apple.com/kb/HT1212
But if it ask for a passcode you have to go to a apple store and they'll restore it with their most powerful macs in the world that don't need passcode verification
If you cannot remember the passcode, you will need to restore your device using the computer with which you last synced it. This allows you to reset your passcode and resync the data from the device (or restore from a backup).
If you restore on a different computer that was never synced with the device, you will be able to unlock the device for use and remove the passcode, but your data will not be present.
You may have to force iPad into Recovery Mode
1. Disconnect the USB cable from the device, but leave the other end of the cable connected to your computer's USB port.
2. Turn off the device: Press and hold the Sleep/Wake button for a few seconds until the red slider appears, then slide the slider. Wait for the device to turn off.
3.While pressing and holding the Home button, reconnect the USB cable to the device. The device should turn on.
4. Continue holding the Home button until you see the "Connect to iTunes" screen. When this screen appears, release the Home button. iTunes should alert you that it has detected a device in recovery mode. Click OK, and then click Restore to restore the device.
Note: Data will be lost. You may have to repeat the above many times.