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Here is an article I would recommend going through when experiencing this issue. If the issue persists after processing these steps, see the section labeled Issue not resolved.
Also, screen curtain may be enabled. Screen curtain is a feature that allows display privacy. To disable screen curtain while VoiceOver is enabled, you can three finger triple tap the screen to disable this feature. You may need to press the home button once in order to process these steps. If you wish to disable VoiceOver, see the link in the article below.