Hello Shirley Ann Fisk,
No joy, hopefully, your phone is currently under warranty. You can attempt holding down the power button for 15-20 seconds to see if you can force the phone to power off.
If the phone gets stuck at the M screen again, I would power it off again by holding the power button for 15-20 seconds then I would remove the SIM card and SD card if there is one installed.
SIM removal instruction:
Power off the phone.
Set the phone face-up with the charging port facing you.
Locate the SIM card slot on the top right of the phone.
Insert the SIM tray removal tool into the small hole on the SIM card tray.
Push the SIM tray removal tool into the hole until the SIM card tray pops out.
Let us know how it goes.