iMore reports that the update that Apple just pushed installed the new version but required users of older iOS devices to enter their Apple ID and password to complete the setup process. You can’t use your iPhone or iPad until you enter these correctly.
An Apple spokesperson explained:
In some cases, if customers do not recall their password, their device will remain in an inactivated state until they can recover or reset their password. For these older devices, we have temporarily pulled back the update and will release an updated version of iOS 9.3 in the next few days that does not require this step.
If you’re stuck with the update installing itself and either can’t recall or enter your password correctly, Apple has posted instructions for a workaround on this page that you can use to restore your device.
iOS 9.3 brings a host of useful features like Night Shift for reducing blue light emanating from your screen, a more secure Notes app, and the ability to pair multiple Apple Watches to a single device.