Generally there will be various reasons, so we will go through them in order. The first thing that you check is the bulb itself and be certain that it is OK. Second is to check all connections: IE the bulb holder, battery connections if system has a battery and switch if it has a switch "some solar light systems come on with Photo-eye switch which recognizes that there is not enough light and they will come on automatic". Third is to check for an open circuit or no continuity. If all of these things check out OK, then you have to get a volt meter and make certain that your Photovoltaic Solar Cell is producing current and output voltage.
