Beowulf is a very long epic poem and one of the oldest-surviving English-language stories. The manuscript has always been in fragile condition and has proven to be quite the challenge to keep preserved.
In 1731, it became damaged in a fire and portions of it were too far gone to be accounted for. One large missing excerpt begins and ends in the middle of the climactic fight with the dragon. It is still possible to follow the story even without the missing piece. Other portions are just pieces of missing sentences, various portions of missing letters, etc., all minor fragments.
Also of note, it is suspected that this story was passed down orally and written later. Because of this, it is impossible to know if the author included all of the pieces of the story.
The missing portion has never surfaced, even in older translations of the work. Later printings either attempt to recreate the missing sections, or include ellipses to indicate the damage. It remains one of the great mysteries of English literature.