1. Natural catastrophe (floods, earthquakes, etc. Most likely not the case)
3. Invasion from outside
The third is the most likely. We know there were groups of people known as the Sea Peoples that invaded both Egypt and Palestine. But how did they defeat civilizations who had been developing warfare for thousands of years? The answer might lie in more effective tactics on the part of the invaders. Not only did they have better armor and weapons, they also had greater fighting skill. The Egyptians, Canaanites, Hittites, Assyrians, and Babylonians all relied on their chariot force to win their battles. They hadn’t counted on a superior force of foot soldiers to outmaneuver and neutralize their war machines. But this is what happened. Chariots didn’t work well in mountains, either, so this is why the Israelites and the Uraurtu (the thorn in the side of the Assyrians) were so hard to defeat in their home territories.
This change in battle tactics was eventually perfected by the Greeks. They created the phalanx – an unstoppable wall of iron-tipped spears and bronze shields – which was imitated all over the ancient world. This was only superseded by the Romans, who had slightly different tactics but more discipline. It was the Romans who saw the final death of the chariot. The last people to use them were the Gauls, Britons and Celts.