Depending on the age, the battery in the device MAY be getting a bit weak, and had lost its charge.
OR - the device went into "sleep mode" (it does that to save power), and had to be connected for a while - to give it enough charge to wake up.
In any case, I would recommend FULLY charging the device, then switch it OFF by pressing the power button for 3+ seconds - so that it switches all the way off. Then wait a few seconds and then try to switch the device ON again (press & HOLD the power button), and then check the battery icon (top-right of the MAIN screen), and make sure it shows that it is charged to a good level.
Please let me know if you have any more questions or issues with the device.
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