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How did you hard restart it ?
You did that part correctly. It is possible that the battery has failed. I want you to reset it from your computer. Connect it to your computer for 20 minutes and after 20 minutes, hold down both Power and Home button at the same time for 20-40 seconds and this will reset it. You may also hear some sound from your computer. Can you try that and let me know the results please ?
Yes, it was probably not getting enough charge. Connecting to computer was to rule out the adapter and force it to turn to recovery mode and also this would charge it.
I'm glad you got it worked.