First, restart your device by pressing and holding the power and home buttons for 10 seconds, until the screen displays the Apple logo.
If after your device has restarted you still can't activate, connect your device to a PC or Mac running iTunes and try updating.
Another way to activate your device is to first turn it off, then log in to the iCloud website with your Apple ID and password. Click on Find iPhone, then All Devices. Select the device you're having trouble activating and click Remove.
Do not click Erase or Mark as 'Lost'. Now turn on your device and try to activate it again. Hopefully that should work, and if it does, go into Settings and turn on Find My iPhone.
If you then see the 'Cannot activate because the activation server is temporarily not available' message, try again later, as it might be that Apple's activation server has shut down due to high demand.
iOS 9.3 freezes when clicking a link in an email
One of the most common problems - which many of our readers have highlighted in the comments below - affects links in emails. When using iOS 9.3, some people have found that by clicking a link in the Mail email app, the iPhone or iPad will become unresponsive.
This solution isn't ideal, and thankfully Apple has pushed out a new update, iOS 9.3.1 which according to the release notes "fixes an issue that caused apps to be unresponsive after tapping on links in Safari and other apps."
Reduced battery life
If you find that the battery life of your iPhone or iPad has noticeably reduced since upgrading to iOS 9.3, try resetting the device by pressing and holding the power and home buttons for around 10 seconds, until the Apple logo appears.
For more advice on improving the battery life of your phone, check out our comprehensive guide on how to improve iPhone battery life.
My iPad 2 has stopped working after updating to iOS 9.3
Many people with the iPad 2 have been complaining that their devices have stopped working after updating to iOS 9.3.
The issues have been serious - and numerous - enough for Apple to release a brand new build for iOS 9.3, known as 13E236.
This update should be rolling out to iPad 2 owners now, so check for the new update. If your iPad 2 is unresponsive - also known as bricked - then you can download the new build of iOS 9.3 by plugging your device into a PC and updating via iTunes.
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Matt is TechRadar's Managing Editor for Core Tech, looking after computing and mobile technology. Having written for a number of publications such as PC Plus, PC Format, T3 and Linux Format, there's no aspect of technology that Matt isn't passionate about, especially computing and PC gaming. Ever since he got an Amiga A500+ for Christmas in 1991, he's loved using (and playing on) computers, and will talk endlessly about how The Secret of Monkey Island is the best game ever made.