One evening, my lights in the living room went out. I checked the breaker and it was tripped. I turned it back on, and now it keeps tripping. This has never happened before.
Could this be a bad breaker?
I have found several times when this happened a light bulb element shorted causing the breaker to trip. It is rare but happens and a good place to look if no other changes to the circuit have been made.
Another possibility is a bad ballast in a compact florescent lamp.
Breakers rarely go bad.
A fault like this could be anywhere in the circuit. In order to diagnose the cause, you will need to isolate the problem. Find out what is hooked to the circuit and unplug everything that is on it. You should also set all of the light switches to the off position.
Once all of the loads are taken off the system, try resetting the breaker. If it resets, then there is a fault with one of the devices plugged into it. Start by flipping on the switches one at a time. If the breaker trips when you turn one of the lights on, then you will be able to narrow down where the problem is. If all the lights come on, then plug in any other devices one at a time to see if that causes it to fail.
If the breaker trips immediately after resetting it, even with everything turned off, then there probably is a fault in the wiring. This could be caused by any number of things such as a loose connection, a nail being driven through it, or by a rodent chewing through a wire. At this point, you may need to get a professional electrician to take a look at it, because there is a potential for injury. There may be a short which could cause a fire.
It could be a bad breaker, or a problem with the wiring that the breaker is protecting you from. You should contact a local licensed electrician, to determine which it is.
If you have the tools and knowledge to troubleshoot this issue, then go ahead and try and locate the problem. However, I implore you to not turn the breaker back on, until you've located and fixed the problem.
Continually flipping the breaker back on to see if it still trips, might be a good troubleshooting technique in software development. But when it comes to things that could burn down your home, this is not the preferred method.
If you don't know how to locate and fix the fault on your own, please contact somebody that does.
In my experience it's usually not the breaker, but it could be. This depends on where you live and a bunch of other factors. For example here in FL the heat in the summer could cause a breaker to become damaged in such a way that it constantly trips once tripped one time. That said your main goal is to safely identify the problem and replace the "broken" part.
First go turn off the breaker, then turn on every thing in the house. See what things are not turning on. Mark down lights, plugs, appliances, and the like.
Next return your house to a normal state.
Then, go turn off all the stuff "on the breaker", unplug all stuff, and turn off all appliances.
Flip the breaker back on. If the breaker trips now, then you may have a faulty breaker, best best is to keep the breaker off, and call an electrician.
If the breaker stays on, go turn on, plug in, and enable all the stuff that was on that breaker's circuit, one thing at a time. If the breaker trips, then were getting closer.
Turn off everything on the circuit again and reset the breaker. This time turn on only the thing that caused the breaker to trip. If the breaker trips again, replace that thing. If the breaker does not trip, then you need to leave that thing on a while. If the breaker still does not trip, then you probably have too much on that circuit. Move things to other plugs that are not on that breaker. Make sure you check that "powerful things" are not sharing the same breaker. For example an old vacuum and portable air conditioner. If the breaker trips after a while, time to call the electrician again, and leave the breaker off. You probably have a wiring problem.
Also you want to watch out for GFCI breakers or plugs. There are some GFCI breakers out there that just don't work with certain appliances. For example I have a water cooler that will constantly trip a GFCI breaker but there's nothing wrong with it. Some appliance just don't work will in GFCI circuits. An electrician can help replace GFCI circuits with normal ones, and can help determine if that's the problem your having.