Ignition breaker Popped wont reset

Hi just doing a warm up temperature impellor flow test using the Bucket method 4-5mins in and I hear a clicky clunk and the engine dies, the temp is fine a little hot (185 ) The Ignition Breaker will not set back in No real audible warning tho there did seem to be a very feint grumble and i mean verrrry feint from the audible warning    There is also a 30A breaker at the back of the Engine it doesnt seem that it is that one as it wont go in . I have decided to let it have a break for 30 mins or so and try again . Last year I did have times when it was difficult to get the ignition breaker to go in.
Any ideas why it popped ????   Thanks