What I am suppose to do

Try performing a hard reset to your device. Simply hold both volume buttons as well as the sleep/wake button at the same time. Your iPhone’s screen should turn black and the Apple logo should reappear on the same screen you we’re before.

If nothing is happening whilst performing a Hard Reset, and the Apple logo is still on the screen, or a Hard Reset was performed and the Apple logo comes back and still is on the screen, try leaving the phone to drain battery. Then once dead, recharge the phone which should power on the phone once battery level is at an acceptable level.

If still nothing works, you may need to perform a DFU update restore to your device.

Instructions to do this can be found here: If you can't update or restore your iPhone, iPad or iPod touch - Apple Official Support

And if you have tried everything and still nothing works, it could be a hardware issue. Have you had your phone previously repaired by a third-party repair store? If so, go to the store and see if they can do anything about it. Remember, Apple will not touch, repair or give you a warranty replacement if you have had a third-party repair, so if the third-party repair stores cannot do anything about it, buy a new phone. Period. End of story.

If no third-party repair was done to your device, then you can simply go to an Apple Store or Apple Service Centre for a check up on the device and a possible repair or replacement if with AppleCare+.

Hope this helps.