If your car is suddenly idling rough, there are a few possible reasons why. The most common reason is that the engine is not getting enough air. This can be caused by a clogged air filter or a blockage in the air intake system. Another possibility is that the fuel mixture is too rich or too lean, which can be caused by a faulty oxygen sensor or mass airflow sensor. Lastly, it could be an issue with the ignition system, such as spark plugs that need to be replaced. If you're not sure what's causing the problem, it's best to take your car to a mechanic for diagnosis and repair.