When Napoleon was defeated leaders from different places sent a message to Vienna for a peace conference, it was to reset the map of Europe, some time in summer kings, queens, princesses, and princes met together.
The first part of it was to deal with territorial issues, and the foundation of a German confederation to replace the defunct holy roman empire.
in March 1815 Neopolitan freed himself from his exile and took back the throne of France and that started the hundred days.
The allies came together and defeated him again at waterloo on June 18th, 1815 and that was 9 days after the last act of the Congress of Vienna was signed.
Basically the Congress of Vienna was a conference held to agree with the settlement of Europe after many wars.