300w 12V pool light with 12 volts at the base and the light will not come on?

This is not a job for a novice. Call a qualified swimming pool tech to trouble shoot your pool light circuitry. It may just be the bulb but age of the light fixture, cord and housing could be a factor also. It may also be just the circuit breaker but unless everything is tested you are just guessing. You are taking a chance, even with low voltage, on stray electricity to the pool. Don't delay getting professional help. k