1.If it won’t turn on then leave it plugged in for a couple of hours, and then hold down the Power button for a good 20 seconds to see if it comes back to life. OnePlus suggests that you try holding the Power button down for 20 seconds five times to power cycle the phone.
2.Try plugging it into your PC and see if it’s recognized or if it charges.
3.Try holding down Power and Volume down and see if it boots into recovery mode.
If nothing works, you may have to repair it.