Before the big hit “Brigadoon” skyrocketed their careers, Lerner and Loewe staged the musical “The Day Before Spring.” Earlier this year Jim Morgan of the York Theater Company decided to bring it back to life, obtained the necessary permissions, but then discovered that the music was nowhere to be found. A detective story of sorts then ensued. A revival in 1990 had been cobbled together from the sheet music of seven published songs from the musical and the recollections of living original cast members and musical directors. But Morgan and his directors found that score to be “so sketchy as to be essentially unusable.” Enter Mark Horowitz, senior music specialist at the Library of Congress. In 1999 the library purchased a set of Loewe’s documents which he had left to a friend who had in turn left it to someone who put it up for auction at Christie’s. Horowitz provided this treasure trove to music director Aaron Gandy who found that “Every song had a different set of materials that survived.” Gandy pieced it all into a full score and this weekend the curtain goes up on a revitalized “The Day Before Spring” as part of the York’s “Mufti” series.