Make sure you switch off the outside light at the wall switch - and take out the bulb for good measure - then try resetting the breaker.
If it still trips then you may have a another bulb (switched on but not working) somewhere else on the same circuit that has failed with broken bits of fillament fallen across the terminals inside causing the trip - not unusual on upwards pointing bulbs.
Switch off and or remove all bulbs on the same circuit (all at once or one at a time or a few at a time) and try to reset the breaker again. If it now trips then perhaps you should call a sparky.
Good luck...take care...
Peccavi
March 2009