rb20 timing belt broke

[rbtank]

well i bought a 89 240sx with a rb20 swap and after driving it a while it would over heat real quick after i hit boost and then when i got to work it sounded like a loud whine and then it jus locked up and i was able to cruise to a parking place. well when my mechanic looked at it he noticed the timing belt was broke, my question now is, what would cause that? and do yall think my motor is done?