This has happened to my 8 sometimes. It happens randomly when multiple notifications come in back to back. So I took my phone to the Apple Store and they replaced the taptic enginge,

watch this video https://www.youtube.com/watch?v=c2iDUJFZ...

If the power adapter was not completely plugged into the wall or console, the battery won't be charged. Try charging the device again to make sure. If your battery is dead, the device will run fine off the power adapter, but will turn off when unplugged. The battery may not accept a charge, the device may not recognize the battery, or the battery may be recognized as charged but fail to power your device.

reboot your phone and try again if the same problem comes then you should change your touchpad