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This sounds like 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.
If the issue persists, see the section labeled Display in the article below for additional troubleshooting steps.