Angel Beats! is an anime that was broadcasted on Japanese TV from April 3, 2010, to June 26, 2010.
While the anime received much critical acclaim upon its original release, there was a small group of people complaining that the end the show seemed rushed - this could possibly be because Angel Beats! was planned as a two-hour (26 episode) series, but it was cut down to 13 episodes at the last minute.
This isn't actually true though. In 2009, there was an interview with the Aniplex producer of Angel Beats!, Yosuke Toba who commented that a 13 episode anime was not enough to tell the entire story.
The anime does have 2 OVA episodes which were included with the DVD and Bluray release, but these are not considered lost media.