If an underwater pool light is not working, you will need to take it out in order to inspect and test it. Your first step should be to ensure the circuit breaker has not tripped. If it has, switch off the pool light and turn on the circuit breaker. If it trips instantly, the problem is a short circuit, which requires the assistance of a professional.
If it doesn’t trip, check the Ground Fault Circuit Interrupter (GFCI). The GFCI is designed to trip if it detects a small current going to the ground; this is a safety measure to prevent swimmers from being electrocuted. You will find the GFCI at an electrical outlet near the pool, near the filter system in a subpanel box, or at your house’s main panel box. If the GFCI has tripped, the most likely issue is water inside the light fixture. If it doesn’t trip and the light stays off, the bulb has probably burned out.