08 f350 6.4l cranks but wont start, cam not turning

[undesputedking]

just bought an 08 6.4 f350 king ranch, i bought it not running, the owner said that it had quit running while going down the road, paul miller ford in lexington ky did a $400 inspection and said that the cam wasnt turning, the motor cranks but it wont start. is it the hpop? if so, how do i fix it? is it cheaper to get a used motor, or fix this one?
please dont reply with: i dont know or hope you figure it out.
thanks

[cdiesel]

Did they say how they found out the cam was not turning? Or if they were not getting a signal to the cam position sensor.