The moon, which is relatively large compared to our planet, is believed to have formed as a result of a collision between Earth and a smaller Mars-sized planet, based on analysis of lunar samples and the similarities in composition between the moon and Earth. So, in a sense, the moon did collide with Earth in the past.
Currently, the moon is not on a trajectory to crash into Earth. It is gradually moving away from Earth at about 1.5 inches per year. The moon plays several important roles, such as stabilizing the tilt of our orbit, causing seasonal changes, influencing tides, and aiding nocturnal animal navigation. If the moon were to suddenly disappear, it would lead to significant environmental changes, and life on Earth, particularly animals, would face challenges in adapting to such drastic shifts. However, due to its slow departure, we do not notice substantial changes.
If, hypothetically, the moon were to crash into Earth, the consequences would be dire for all life. To put it into perspective, the asteroid that caused the extinction of dinosaurs was approximately 12 km in diameter, whereas the moon has a diameter of about [2,174 miles] 3,500 km — roughly 300 times larger. However, there is a concept called the Roche limit, which represents the minimum distance at which a large satellite (like the moon) can approach its primary body (Earth) without tidal forces tearing it apart. At approximately 11,500 miles above Earth, the gravitational forces from Earth would overpower the gravitational forces holding the moon together, causing it to break apart. Over time, the moon's debris would rain down on Earth, burning in the atmosphere and leading to a warming climate beyond what life could survive. Ultimately, this catastrophic event could result in the extinction of life on Earth, except perhaps for some extremophiles.