Hm. I must suggest two more thing. As the last thing we want is to have to factory reset.
Hold the Power button AND Volume Down for about 10 seconds.
If that doesn't work, wipe the phone cache:
Turn off the device. Connect to a computer or plug in USB-C headphones. Press and hold the Volume Up key, then press and hold the Power key. When the Android logo displays, release the keys. A message will show for 30–60 seconds until system recovery menu options appear. Press the Volume down key several times to highlight "clear cache". Press the Power key to select. Press the Volume down key to highlight "yes," then press the Power key to select. When the clear cache partition is complete, "Reboot system now" will be highlighted. Press the Power key to restart the device.