For what it's worth, any time I have the oil cooler removed on one of these engines, I always like to remove the high pressure oil pump as well, leaving the gallery under the oil screen completely open. Using a blow gun to blow all ANY foreign debris that could possibly have made its way in there and through to the IPR valve to avoid this exact scenario. My advice to the original poster is to remove the IPR valve for a look/see. If the screen is trashed, the IPR valve needs to be replaced. Failing that, you'll need to access the typical 6.0L no-start PIDs that I'm sure you'll find very easily in this forum with some searching, to see what's missing.
If the IPR valve is in fact trashed, this post should be a perfect testament as to the importance of clearing the oil cooler area completely free of any dirt or debris prior to its removal, to ensure none of it makes its way into the reservoir.