It seems that the recently released iOS 9.3 update has been problematic on some older devices which have stalled in mid-update and effectively left those devices useless. During installation, the update requires the Apple ID and password originally used to set up the device - the common problem is this ID might not match your current one. Apple has responded by releasing a series of workarounds found here. Apple will be issuing an update to the update to fix the issue affecting older iPhones, iPads and iPod Touch devices.