How did charlemagne briefly reunite much of western europe, and what happened to his empire after his death?

Charlemagne belonged to the Germanic tribe, the Franks. Upon the death of his father, Pepin, and his brother, Carloman, Charlemagne began his campaign to unite all the Germanic peoples/ tribes into one kingdom. After uniting the Germanic tribes, he had them convert to Christianity.

Next, Charlemagne aimed to expand his kingdom, carrying out successful military campaigns against the Lombards (in modern-day Northern Italy), the Saxons, and the Avars (in modern-day Austria and Hungary). He conquered most of Western Europe reaching Northern Spain, Bavaria, and Southern Italy. 

Charlemagne employed "novel siege technologies and excellent logistics." He led a well-organized large army, heavily armed and armored, who traveled in horseback, allowing them to travel large distances. They relied on their maneuverability and on an organized system of conquest, building fortresses, leaving garrisons in forts, and gathering enough resources and supply for their conquests.

As an emperor, he was a talented diplomat and converted most of his kingdom to Christianity. He initiated military, economic, educational, and religious reforms, making him the protagonist of the "Carolingian Renaissance."

After Charlemagne's death, his son Louis became the sole ruler of his empire. Eventually, his empire was split among 3 of his grandsons and was dissolved by the late 800s, primarily because of a new wave of attacks from the Muslims, Vikings, and Magyars.