B737-500 flight crew encountered a failed landing gear extension on approach. Performed a go-around and resolved the problem by resetting a popped air\ground switch circuit breaker.

Narrative

Extended landing gear approximately 2 MI prior to the runway. With gear handle down landing gear panel indicated one red nose gear light only. All other lights out. Attempted to recycle but gear handle could not be raised to up. Moved throttles to idle with 10 degrees of flap and gear warning horn sounded. Executed a go-around at approximately 1200 FT AGL; reaching assigned altitude of 9000 with approximately 4100 LBS of fuel advised ATC that we were now min fuel; as we believed any delay would result in an emergency fuel condition. With aircraft now at a safe altitude we checked the circuit breakers and found the air ground relay and lights circuit breaker out. We reset the circuit breaker; raised and lowered the gear handle and received a normal 3 green indication. Not knowing why the circuit breaker had tripped we elected to leave the gear down while maneuvering for another approach. Due to the high fuel consumption and heavy volume of traffic we declared a fuel emergency. We advised dispatch of our fuel emergency via ACARS. With expedited handling we landed with 2900 LBS of fuel.