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okay, try this please.
Hold the power button down for 30 seconds, then wait one minute and press the power button to turn on and press and hold the command and R keys at the same time .
\When the disk utility screen appears, choose restore.
Let me know if this does not work and we can try another solution.
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okay, power off again. then press power and hold down the shift key while it is powering on.
okay, follow the steps in this link please>https://support.apple.com/en-us/HT201314