If you can't unlock the device because it will not accept the passcode, there is only one thing you can do.
You have to restore the device within iTunes. You want to use the same computer that you always sync with so that you can restore your app data and settings. You can restore with any other computer, but you will lose everything on the iPad.
Instructions can be found here.
You may have to use recovery mode if that didn't work.
BUT .... Just for the heck of it, reboot the device before you resort to restoring and see if ithe passcode will work.
Reboot the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider - let go of the buttons.