seized 440 need help

my uncle has a chrysler newport. 1969. he has put a 440 in it, engine all connected and was running fine. a hydraulic lifter was tapping, so my uncle changed the oil, and put duckems in. started fine, and was running fine, came to start it again, and wouldnt turn over. so were pretty sure its seized

but why should it seize just all of a sudden. oil pressure light was on.

any advice would be helpful. dont no alot about the 440's

cheers