Short answers, pulled from the story.
The destruction of Napoleon's Grande Armée during the Russian Campaign in 1813 created a power vacuum that Prussia and Austria could not ignore. These two states had been forcibly allied with France after earlier defeats, but the collapse of their enemy allowed them to pivot against him.
Prince Schwarzenberg led the Army of Bohemia with 200,000 to 210,000 Austrian soldiers through Swiss territory. Blücher commanded the Army of Silesia containing 50,000 to 75,000 Prussian and Russian troops crossing between Rastadt and Koblenz on the 1st of January 1814.
Napoleon struck at Blücher's scattered detachments during a rapid series of battles from the 10th of February to the 14th of February 1814. By the 14th of February at Vauchamps near Étoges he crushed Blücher's main body.
Tsar Alexander I and King Frederick William III decided to ignore Napoleon and march directly to Paris instead after a letter outlining his plan to move on Coalition lines of communications was intercepted by Cossacks in Blücher's army on the 22nd of March. Marmont and Mortier took up positions on Montmartre heights but surrendered the city on the 31st of March when further resistance proved hopeless.
Napoleon signed the Treaty of Fontainebleau on the 6th of April 1814 after refusing initial terms regarding his son. He was sent into exile on the island of Elba while Louis XVIII restored the Bourbon monarchy.