My daughters Samsung Galaxy Tab A has suddenly changed her lock screen pin number to a password which we do not know.   It asks for a password containing a minimum of 4 characters and one number, it previously was a pin with numbers only. 

It also won't allow us to turn the device off or restart without the password.  I have forced a reboot but can't see another way to turn the device off in order to do a factory re-set. 

Any ideas on how to fix?

Thanks.