[Snoonyb]

You probably have water in the conduit which has caused a deterioration in the wire insulation and a subsequent short.
Since you are also probably unfamiliar with working behind a "dead front panel", absolutly turn the power off at the main before you attempt anything inside the electrical service.

You can test this by using another breaker in the panel, however, everytime a breaker fails because of an electrical problem in the connected circuit, the devise is weakened, and its useful life shortened.

You'll need an inexpensive VOM (Voltage, Ohm meter).

To check the breaker;
1) With the MAIN disconnect turned off and the breaker in question turned off, not failed, remove and seperate the conductor from the side of the breaker.
Reset the breaker to the on position and turn the MAIN service disconnect on.
2) With the meter in the voltage scale and the range set to a scale higher than 200V, test for the presence of voltage at the screw on the breaker to the neutral buss bar.

If voltage is present (120V), the breaker is working.

To check for a short;

1) With the main service diconnect turned off.
2) Turn off the breaker in question and test for the presence of voltage at that breaker.
3) Disconnect the conductor from the breaker, the corresponding white or neutral conductor from the neutral buss bar and if there is one , the ground conductor, seperate.
4) In the garage, open the "J" box, disconnect and seperate the incoming conductors.
5) With the meter set on the Ohm scale test all of the conductors to the conduit,DO NOT, use your fingers to pinch the conductors to the meter probs.
6) Next, with the meter still on the Ohms scale, test the colored and white conductors to the ground conductor and then to each other.

If there is a reading from the colored or white conductor, either to each other and/or to the ground and/or conduit, you have a short.