How can you tell if your door bell is done working? And if so are they hard to replace? I am a single woman?

We need more to go on. Much of how hard to change depends on what it is inside of it if anything. Likely it has a couple of small screws holding it in place and a couple of wires, usually only 24 volts. It is possible just opening it up and squirting it with some spray lube will return it to the normal sound. I prefer a silicone to whatever WD-40 is. Of course, describing sounds is tough.