Lincoln's assassination was a really big deal because he was the President of the United States during a time when the country was trying to come back together after a terrible fight called the Civil War. When he was killed, people worried about what would happen next.
You see, Lincoln had big plans to help heal the country and make sure everyone, no matter their skin color, would be treated equally. After his assassination, many people were scared that everything Lincoln worked for might fall apart. There was a lot of anger and sadness, and some people even thought that the country might break apart again!
Also, the person who became president after Lincoln, Andrew Johnson, had different ideas and wasn't as popular. So, people were not sure if he could lead the country in the right direction. The fact that Lincoln was gone made everyone worry about whether the nation could stay strong and united, which is really important for peace and safety. His death was like losing a captain of a ship in a storm; it made everyone uncertain about where to go next!