Generally, damage to an automobile will be covered by comprehensive car insurance, which is in addition to collision coverage that applies to a car or truck while it is in use. In addition to accident insurance, comprehensive insurance will generally cover damage to an automobile that didn't stem from a driving accident.
This type of comprehensive car insurance can cover a wide array of damages stemming from natural disasters—such as tornadoes or hurricanes—theft and vandalism, and falling objects, the last of which includes a garage collapsing on an automobile. Similarly, a tree falling on a car would be covered under this comprehensive car insurance.
The damage to the garage is most likely going to be covered by your homeowner's insurance. As with any insurance, it doesn't hurt to talk to an agent or insurance company to confirm your coverage and the premiums to be charged. For instance, all-terrain-vehicles (ATVs) might be covered by homeowner's insurance while parked in a garage, meaning they would be covered if a garage falls and damages them. However, this will most likely not apply to automobiles, motorcycles or boats, which have their own coverage in the form of comprehensive car insurance.
Car theft in a garage is another tricky scenario, but it should also be covered by the comprehensive auto-insurance policy.
Obviously, multiple possibilities arise when insuring a home and its contents, including an automobile in the garage. Talking specifically with an insurance agent is going to be the best way to ensure coverage will be provided in case an accident occurs. (For related reading, see: What Is and Isn't Covered by Homeowner's Insurance.)