Do you mean you can't get into Restrictions, or that you can't change the setting?
If the first, it's almost impossible to bypass the restriction code. Remembering it, or restoring the iPad is pretty much the only option.
If the second, try turning Restrictions off altogether, then use Settings > General > Reset > Reset All Settings. This won't delete anything, but it does default most settings.
Before you do either of these, you can try the usual fixes; if you haven't already.
That's all I can think of, at least without more information.
Good luck.
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The Usual Fixes
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When you have problems with an app it's a good idea to clear it from RAM.
Close/Clear the app:
iOS 6: With the problem app closed, double tap on the Home button to get the multitask bar. Tap and hold on any icon in the bar until they jiggle, then tap the minus sign on the problem app.
iOS 7: Double tap the Home button to get the multitasking display. Swipe up on the problem app's thumbnail image (not the icon).
If that does not work try a restart,
Restart: Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears. Apple recommends you restart the iPad at least once a month.
and then a reset if that does not work.
Reset: Hold down the power and home buttons until the Apple logo appears. The iPad will reboot. This usually takes a bit longer than a restart.
Reset All Settings: Another, somewhat more drastic thing you can do is Reset All Settings. You'll find this under Settings > General > Reset > Reset All Settings. It won't erase any of your content, apps, or accounts; but you will have to go through most of your other settings and re-tweak them back to your liking.
Do not confuse this with Erase All Content and Settings.
More detailed information at:
http://support.apple.com/kb/TS3281?viewlocale=en_US&locale=en_US