Not familiar with the brand but I'm guessing from your post that this light is supposed to come on automatically when it gets dark?
If that is the case then the photo
sensor and solid state switch are almost certainly in the light itself. However it also needs to be powered on from the panel or switch below.
First, always check the easy stuff first. Make sure the below decks circuit that sends power to the light is always switched on (make sure it isn't powered by some circuit that gets switched off and on for something else). Also make sure you're getting good power to the light.
If that end it good then you probably have a problem at the masthead. Either a bad wiring
connection to the light or a failure in the light itself.