once in event viewer, go to the submenu System and you could either look around in a chronological order, or order them by message type (left click on "Type" tag) and look for any error or warning message that has any reference to a forced/unexpected shut down, sadly the info sometimes isn't really obvious so you'll have to read any error/warning message that is around the hour the shutdown happened (or that belongs to the night before if you don't know at which hour it happened.)