Does it turn off, or simply enter sleep mode?
If so, you can change when it sleeps by choosing Settings -> Auto-Lock -> and select a time.
Also, check memory available. If it gets below 1 gig or lower like 500 mega, better start deleting ebooks, apps or games/programs etc.
- Plug your device into power and connect to the Internet with Wi-Fi.
- Tap Settings > General > Software Update.
- Tap Download and Install. If a message asks to temporarily remove apps because iOS needs more space for the update, tap Continue or Cancel. Later, iOS will reinstall apps that it removed. If you tap Cancel, learn what to do next.
- To update now, tap Install. Or you can tap Later and choose Install Tonight or Remind Me Later. If you tap Install Tonight, just plug your iOS device into power before you go to sleep. Your device will update automatically overnight.
- If asked, enter your passcode.
Use this guide:
press and hold down the Sleep/Wake button and the Home button at the same time.
Keep holding down the button combo until you see an Apple logo appear on screen (your iPhone screen will go dark before the Apple logo appears). iOS devices with white faceplates will show a white launch screen. iOS devices with black faceplates will show a black launch screen.
When the launch screen appears, you can let go of the button combo.
Once rebooted, check to see if it stays on and operational.
If that doesnt help you in any of those areas, you may need to reset.
- Connect your iOS device to iTunes on your Mac or Windows PC using your device's charging cable.
- Click on the Device button at the top left.
- Click on Back Up Now.
- Wait for the backup to finish.
- Click on Restore iPhone… (or whatever iOS device you've connected).
Wait for the restore to finish.
I can send you tutorials on these if you request them or if they are needed.
If these fixes do not work, that means you may have a hardware issue and it needs to be serviced.
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