When you try to reset the breaker, does it trip again right away?
If so, you've probably got a short circuit somewhere.
The breaker is doing what it should to prevent a fire.
You may need a mechanic, but you can try turning off all the 12V loads.
If you can then reset the breaker, the short is associated with one of the things you turned off. Turn them back on one at a time, and you might be able to narrow down the culprit.
The water pump motor is one possible load that might fail shorted.