Steps to follow  if iOS 12 won't restart or let you reboot:

• Press and fast release the volume up and down buttons, then press and hold the side button. Release the button when the Apple logo displays.
• If your phone's battery is actually dead, try connecting it into a charger—it won't always turn on immediately away. Before turning it on, try leaving it plugged in for 15 to 30 minutes. If that doesn't work, your charger can be damaged. Change the cord, power bank, and wall outlet if necessary.
  
  
  
  
  
  
  

Read More: How to troubleshoot Bluetooth connections