Napoleon's empire collapsed because of the Continental System, the Peninsular War, and the invasion of Russia. These events are widely viewed as three major mistakes that led to the weakening of his rule in Europe.
The Continental System (1806) was a trade embargo Napoleon introduced between Great Britain and Europe. He wanted to weaken Britain's trade system. However, the Continental System backfired on Napoleon as it didn't affect Britain significantly and other Eurpean countries continually ignored the embargo.
The Peninsular War (1808) began when Napoleon attempted to force Portugal to accept the Continental System. Napoleon invaded Spain and replaced its king. This angered Spanish nationalists, who sent troops to war with the aid of the British. Napoleon lost 300,000 men.
The Invasion of Russia (1812) secured Napoleon's downfall. He sent troops to invade Russia, who had refused to end trade with Britain. Russia used the "scorched-earth" tactic to starve Napoleon's troops, defeating the French army as they turned homeward, weakened from hunger.