Abraham Lincoln died 150 years ago yesterday. That is, he died on April 15, 1865. He died at 7:22 that morning after having been shot at about 10:15 the previous night. He died from the effects of the gunshot wound having never regained consciousness after being shot. The man who killed him was a famous actor of the time named John Wilkes Booth.
Booth shot Lincoln while Lincoln was attending a play at Ford’s Theatre in Washington, D.C. Booth was born in Maryland, which was a “border state” where slavery was legal but which did not join the Confederacy. Like many people from Maryland, Booth was a Confederate sympathizer. As the Civil War was coming to an end, he first hatched a plot to kidnap Lincoln as a way of allowing the South to win. When that did not work, he plotted to assassinate Lincoln and other high government officials to throw the US government into disarray and save the South. As it happened, only Lincoln was killed and no attempts were made on the other officials.
The short answer to your question, then, is that Lincoln died from a single gunshot to the head. He was killed by John Wilkes Booth.
John Booth Wilkes was a famous actor at that time. He and a group of friends plotted to attack many high ranking officials. While the president was watching a play, Lincoln's security guard went to a bar, giving John a chance to shoot Lincoln while he was unaware.