Two myths about cats need to be dispelled immediately. The first is that cats come trained for living in a home. False! Cats have instincts that make them do things we like. But they are not trained to do those things. The exact same set of instincts that make your cat so easy to live with can also make her hard to live with. A small change to your home can trigger the wrong instincts and cause problems.
The second myth is that cats can not be trained. False! Cats are smart and learn many things about you. They learn your schedule and moods. They can learn many other things as well. They just need a good teacher.

Never confuse instincts with training. Instincts are always with your cat and are very difficult to change. Instincts can be fragile in many ways. Training are the behaviors that you choose and reinforce. You can make training very reliable even with distractions and changes in your home. Reinforcing all good behavior is a way to pile training on top of instincts.
There are three important components to changing a behavior problem. The first is knowing your cat and taking her to the veterinarian when ever there is a change in behavior. Cats don't tell you when they are sick. They try very hard not to let you know. Watching and knowing when something isn't usual is important when describing the symptoms to your veterinarian.
The second component is environment. Cats reactive instinctively to their environment and even a small change can trigger a very different behavior. Think about what has changed lately. Also consider changes outside like a new cat roaming the area around your home. New noises like construction on your home or near by.

Once you identify the change you can think about how to trigger the correct instincts and get the behavior you want back.
The final component is actual training: Reinforcement for good behavior and behavior that replaces the things that annoy you. Training a couple fun tricks now before a problem exists is a good basis for needed corrections later and is just good for your cat overall. Cats require 100% positive training like the popular clicker training method.