Q: IPad Air screen freezes

Hello there, Air3535.

It sounds like your iPad stops responding after you updated your iOS to 8.4. Although this Knowledge Base article is written for apps installed on your iPad, it offers some great initial steps to try:

If an app you installed unexpectedly quits, stops responding, or won’t open - Apple Support

Restart the app

Close the app and open it again.

Restart your device

Turn your device off and back on.

Check for iOS and app updates

1. Make sure that you're using latest version of iOS.
2. Open the App Store and tap Updates.
3. If updates are available, tap Update All.
4. If asked, enter your Apple ID password. 

Additionally, this article provides some more clarification:

If your iPhone, iPad, or iPod touch doesn't respond or doesn't turn on - Apple Support

If your screen is black or frozen

First, try to force your device to restart.

• Forcing your device to restart won't erase the content currently on your device.
• You can force your device to restart even if your screen is black or your buttons aren't responding.

Press and hold the Sleep/Wake and Home buttons at the same time for at least 10 seconds until you see the Apple logo.

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What to do next

Contact Apple Support to set up service if you have any of these issues:

• You still see a black screen after charging, force restarting, or following the steps in this article.
• You see a black screen, but can hear alerts, sounds, and vibrations.
• Your screen is on, but it doesn't respond to when you tap, slide, or try other gestures. 
• You can't force your device to restart because your Home button or Sleep/Wake button is broken or stuck.
• Your device is stuck on the Apple logo, displays a solid color, or isn't recognized in iTunes.

Thanks for being a part of Apple Support Communities.

Kind Regards.