2001 Mazda millenia that has had a bad knock for 6 months now the car starts and runs but once it is shut off it will not start again Why?

do a compression test..possibly serious...knocking for 6month is very hard on an engine

This needs to be looked at by a professional and needs to be looked at 6 months ago. You have been driving this car in this condition for 6 months? A knock is normally associated with worn bearings. Keep driving it and it will self destruct.