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It is disabled because someone had entered incorrect passcode for number of times. It will be disabled completely if incorrent passcode is entered for multiple time.
Unfortunately your only option is to force-restore your iPad to factory settings through iTunes by putting it to DFU (Device Firmware Upgrade) model, which will erase everything from it. This is how Apple designed it to protect the information of their users. There is no other way to enable it again.
If you've backed up your iPad to iTunes on your computer or iCloud, then you can restore all your data back after restoring your iPad from iTunes/iCloud. You can also redownload all your past purchases for free using the same account.
Follow the steps below to restore your iPad by putting it to DFU Mode.
First download and install latest iTunes on your computer from here : https://www.apple.com/downloads/
- Connect the iPad to your computer with the USB Cable.
- Open iTunes application on your computer
- On your iPad press and hold the Home button and the Power button at the same time.
- After EXACTLY 8 seconds release the Power button. Continue holding the home button until your iTunes shows a message telling you that it has detected an iPad in recovery mode.
- Click OK or Restore
Note : Put it to DFU Mode while connected to computer and also in DFU Mode your iPad will remain blank. Timing is critical when you put it to DFU Mode. So keep trying until it goes to DFU Mode
Please let me know if that didn't work, so we can continue troubleshooting.