1. Broken timing belt
Probable causes: 1) Straight cord line rupture: belt was crimped before fitment; 2) Raffled cord line rupture: foreign object between the belt and pulley will break the tensile cords; or belt was twisted over 90° or turned inside out before fitment, or forced on a pulley e.g. using a sharp object such as a screwdriver.
Corrective actions: Make sure to observe proper belt handling instructions and never coil, crimp or twist timing belts as this damages its tensile cords; Install at correct tension.