Can be tons of reasons, here is some based on how common they are by my expiriance:[br]
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1. Bad battery that causes some electricity anomaly. Try turning it with charger only[br]
2. Bad charger. Can be tested with simple multimeter.[br]
3. Some motherboard malfunction. Fairly hard to test unless you have a spare one.[br]
4. Broken power button/switch? Usually rare but it happens.