What happens if you don't use your car for a while?

The battery can run down and the fuel can start deteriorating and there can be condensation in the engine and gas tank if the humidity is high. If you plan on storing a car for months I would use a good gas stabilizer like Sta-Bil, fill the gas tank and disconnect the battery or use a battery tender. Changing the oil would also be a good idea. If your tires tend to flat spot then jack the car up and put it on blocks.