Music styles change, tastes change. Also, stuff just gets misplaced. Antoine Brumel (15th-16th century, ecclesiastical composer) has a number of surviving works, but the one considered his masterpiece only survived in fragments. In the late 1980s, a very large lost fragment was found between the pages of some old book in Denmark (!) and reunited with the parts that had not been lost. A little remained to be found, but the modern arranger made up something, and the entire work (with the modern bridge) was performed for the first time in perhaps 400 years.
I read about it fifteen years or so ago, had to special order it, and it's easily one of my favorite CDs. There's a new, remastered version of the recording now, plus it has been recorded by at least one other ensemble.
Antoine Brumel:
Missa 'Et ecce terrae motus' and
Sequentia 'Dies irae, Dies illa'
by Huelgas Ensemble
