What brought about Napoleon's downfall?

Like many similar questions, this could have a short or a long answer. The long answer might involve an analysis of the long-term feasibility of conquering and ruling a multi-national continental Europe without possessing significant naval power to counter Great Britain. If one finds that prospect to be dubious, then one might answer that Napoleon's efforts were doomed from the start, or at least from the moment that Nelson defeated his navy at Trafalgar.
A short answer might focus on the immediate details of Napoleon's defeat. Perhaps the most significant element in this line of inquiry is Napoleon's disastrous invasion of Russia in 1812. The heavy casualties at Borodino and the conquest of a burned out Moscow proved to be a pyrrhic victory at best. Napoleon invaded Russia with around 450,000 troops, and returned with only a fraction. While Napoleon once boasted that he could "spend 30,000 troops in a month", such casualties could not be sustained forever.
The defeat in Russia significantly weakened Napoleon's position, and in 1814 he was defeated and forced into exile on the Mediterranean island of Elba. Making a daring escape, he returned to France, only to meet a final defeat at Waterloo, spending the rest of his life on the remote island of St. Helena.
Fighting multi-front land wars against large coalitions would be a daunting task for any military commander, including Napoleon. This challenge, paired with the successful British naval blockade, brought about Napoleon's downfall.

Napoleon was an extremely successful general until his defeat in Russia in 1812. His defeats on this front marked an end to his very successfull campaigns and meant he reconsidered his advances in the next couple of years on the European front. Ultimately he was defeated in 1815 at Waterloo by a coalition of British forces and the armies of the Prussian Emperor. These forces pushed Napoleon into a life of reclusion on the island of Sicily, from where he requested freedoms on July 13, 1815 in a letter to the Prince Regent. Napoleon called upon the hospitality of the British people and begged that they reconsider his position as exiled ruler of Europe. He hoped that he would be welcomed back to rule his people, but was refused and died in exile.