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If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open - Apple Support
If your top (Sleep/Wake) button is unresponsive, it may be broken - and if so, will ultimately render your iPad unusable. It is a common misconception that an iPad (or iPhone) can remain fully functional with damaged or defective physical buttons.
Whilst Accessibility features (such as assistive touch) can be used to augment the User Interface, these features cannot be used as a substitute for physical button sequences that must be used to trigger system recovery actions. Sooner or later, you may encounter a situation from which you cannot recover.
If needed, you can arrange repair of your iPad either by (a) visiting the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - or (b) using Apple's mail-in repair service:
Genius Bar Reservation and Apple Support Options - Apple
iPad Repair & Service - Apple Support