Main breaker will not reset

Also check all the neutral connections. They seem to come loose more often than the "hot"wire. I remember having an issue in Vegas with my main breaker tripping. Attributed it to really hot weather and ac running hard. Then breaker did not reset after a few trips and my power cord was very hot(hard to judge in the vegas sun). Turns out the neutral connection at my park "service" was loose. After they changed out the receptacle my ac was running much better and no more trips.