Why there is not good answers to this stuff on the Apple Forum I don't know, I had the same issue and found this online (http://www.macworld.co.uk/how-to/ipad/how-activate-old-ipad-after-update-3638936 /) and it worked for me:
1. Plug your iPad into your Mac or PC. (Interestingly, Apple acknowledges that some iPad owners might not actually own a PC or Mac, and advises them to visit an Apple Store or authorised reseller and use their facilities. Not a great solution if your nearest outlet is miles away, of course.) At this point it would be a good idea to back up your iPad, either locally or to iCloud, but if your device is totally unresponsive you may not be able to do so.
2. Open iTunes, and check you've got the latest version. (At time of writing that's iTunes 12.3.3.)
3. Force your (still plugged-in) iPad to restart by pressing and holding the power (Sleep/Wake) and Home buttons at the same time. Keep holding them down until you see the Recovery mode screen. You probably need to keep holding these down for longer than you think: the screen will go black then after a few seconds, and the Apple logo will appear, but you need to keep holding the buttons for another 10 seconds or so before the iPad goes into Recovery mode.
4. You'll be given the option to restore or update in iTunes. Select Update.
5. iTunes will now try to reinstall iOS "without erasing your data", Apple says, reassuringly. At time of writing, the version of iOS that will be installed is iOS 9.3, but don't worry - it shouldn't cause any problems this time. (Well, Apple admits that this step may not always work. It advises that if the update process takes more than 15 minutes, your iPad will exit recovery mode, and you'll need to repeat steps 2 and 3.) One user reports seeing the error message again during this process, but says you should ignore it and wait.
6. After the update is finished, leave your iPad connected to the computer. You'll now need to complete the activation setup process for your iPad using iTunes.
7. After iOS 9.3 is done downloading, you will need to repeat step 3. This time your iPad will ask if you want to restore it.
8. Once the restore is finished, you will be asked to select whether you want to activate the iPad as a "new" device or if you want to restore it from your last backup. Selecting the new iPad option will format your iPad so it is like new and restoring it will retrieve your data again.