Need help to unbrick JIAYU G3

I have a jiayu G3 with the needrom android 4.1.2 ROM, I wanted to give it to my son and thought factory reset is a good idea...I left the phone to it, it rebooted and the little robot was busy. When I cam back all that was on the screen was the JIAYU logo, the bootloop and the battery was close to empty although it was connected to a charger. I can get in to the factory settings by pressing the volume down and power button. I can switch between the different modes but I can not chose any of them, the power button just seems not to work. I tried all sorts, connected to a charger, to a computer (Mac) , makes no difference at all, can anybody give me a tip where to take it from here? 2 hours ago it was a (nearly) perfectly working phone, now just a paperweight....
Cheers
Jo

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