TMZ had these details: According to the band's rep, the Boys had just finished their set when singer Henry Garza tried to greet some fans, but misjudged his step and toppled off the stage ... falling right into the orchestra pit.

We're told Henry's bandmate and brother, Joey Garza, immediately jumped down to his aid ... and Henry was transported to a local hospital.

The band was forced to cancel three performances in Hawaii this weekend as a result of the fall.