What does it mean when my 99 cummins 24 valve will turn over but won't start?

It means you need to get it diagnosed to determine the cause of the problem. Maybe you're not getting fuel (lift pump failure is a common occurrence in the 6BT), maybe you're not getting compression in your cylinders (which could be for various reasons), maybe it's an ECM issue. There's more than one possible reason why this could be happening, and we can't tell you definitively what it is without a hands-on diagnosis.