He was crowned Holy Roman Emperor in the 800s. But before that, he had to conquer kingdoms. By the time he was given that title, his empire consisted of France. It also included Germany, Austria, Italy, and northern France.
However, when Charlemagne died, the empire began to weaken. His successors weren't as successful as him to rule the empire.
As power monarchy had declined, nobles turned to defending their own lands. While they still were loyal to their king, they ruled their land separtaely.