Most likely it is NOT the breaker. Why does everybody assume the breaker has suddenly gone defective? You likely have a loose connection early on in the circuit.

I can't speak for everybody, but I assume it is the breaker because it is the only part of the electrical wiring that I (sort of) understand.

The problem is that anybody who knows about this stuff realizes that the breaker is the LEAST likely part of system to go bad. It is ultimately the device that is responsible for the safety of your electrical system and as such there are very tight specifications governing it. They are really well made and very unlikely to fail.

But replacing them seems to be the inexperienced person's first thought.

As Ron said, it's most likely a loose connection somewhere on the circuit. Start opening up the boxes and inspecting.

The most common fault I've found has been bad continuity in back-stab devices. Just by opening the boxes and extending the devices, continuity is usually restored when the wire rotates a bit in the connection but it's a good idea to at least rewire them to the side screws, or better yet, replace the device entirely.

Thanks for the input to all of you. I will look into it.