Won't boot any more

Hi, first: please excuse my bad english. Yesterday i updated my old Imac. When the update was nearly completed, but the Mac not yet rebooted, unfortunately I started to clean the scrollball of the mightymouse with a wet cloth. suddenly the computer turned off. Now it doesn't start anymore. When I push the button on the backside I hear a short noise of the harddrive as if it would turn a few times, sometimes only a very low click. Is it possible that I produced a short-circuit on the USB-supply? Or maybe I turned off the computer accidentally when cleaning the mouse and the uncompleted update is the reason? Is it necessary to open the mac or is there another way?

Thank you for your help!