I'd wager you've collapsed a lifter due to starvation.. it's been known to happen with heavier oil in cold environment, and especially if that oil has some age on it (lots of ash)..
there are some ways to recover a lifter, but they are flimsy attempts at best.. one is to run a really light oil in effort allow it to charge.. one is to run seafoam in the oil to break up debris/deposits that may be blocking the tiny port on lifters.. another is to pull the valve covers and manually depress the lifter by pressing the push rod, and allowing it to slowly rise.. yet another is attempting to fill the lifter via hollow push rods.. all, like I said, are flimsy at best..
the only way to make it happen with certainty is to replace the lifter.. SOME head designs allow the removal through the heads.. I don't know if these can be yanked out like that or not- you'll have to wait to see what others more versed in the 6.4 say.. most often, there is a 'spider' plate that locks the lifters in the valley, and the intake has to be pulled off to get to them.
I would 'try' to run some lighter viscosity oil through the engine to attempt to re inflate the lifter... you may get lucky!