Before we scare you with the possibilities of a HPOP (High Pressure Oil Pump) failure, you need a scangauge or equivalent to monitor and or read what some of your sensors are showing. The primary sensors to be read with a crank no start on a 6.0l diesel are:
ICP - Injection Control Pressure
IPR - Injection Pressure Regulator
What the 2 sensors are reading before and during cranking will provide alot of information.
The FICM (Fuel Injection Control Module) Has 3 areas that need to be monitored at all times, they are:
Reading here will give you some helpful information:
Troubleshooting a "no Start" condition