For Built-In Batteries try to Press the power botton and the Volume(-) at the same time. It will help for a hard reset . My Phone Overheated and wont turn on. Mine is O plus brand and it is a Buit-In Battery. What Could a Possible causing your phone Overheat? 1. Using it while charging.(Not included using Power Bank) 2. Charging your Phone - Drain or 20% Up. ( Charging it for too long will Overheat your phone too.) 3. Leaving it in a Hot place. 4. Not Shutting down all running apps.( It will take percentage even if your phone is off) I Hope this Help. Share your Thought in the comments below and make sure to answe YES , as a Thank You.