how to recover forgotten restrictions passcode

Only you can remember it. If you can't remember the restrictions code then was it on the iPad when you last backed up ? If not then you can restore to that backup and it should be removed. If it was on the iPad when you last backed up then the only way to remove it is to reset the iPad back to factory defaults and you can then re-sync your content back to the iPad - you won't be able to restore to the backup as that will keep the code in place.

Have you tried connecting it to your computer's iTunes and resetting via that ?