Maybe the phone was updating and you brute force rebooted it.
Phones have become complicated like computers, the phone was lagging because it was doing something for sure then you shut it down.The Phone wasn't updating, many people are having this problem, me being one of them, here to the phone started lagging, so decided to reboot, and then It won't turn on. Tried all the tricks, ended up waiting 3 days and put it in de charger, after 5 minutes I I kept the power button pressed and it gave a sign of life, but showed the battery empty, waited 5 more minuted and tried again. This time it turned on, after a week exactly the same problem, after 3 days I could start it with the same procedure, at this time there was an update, installed the update, and tried to restart, it now turns off, does start, but I don~t have to wait dthe three days., after some time I can start it manually again, but it is a clear problem with the phone. ANd I expect it to be software related.
Your option was to "hard reset" the phone and that is what any repair shop will try as a first step.Ok, last time I succeeded in turning on my phone, I went I to updates and feedback and send message with the logs of my phone. Today I got a reply, they have determined that it is the motherboard and they adviced me to take it to the nearest deviceocation and have it checked out