If you bent a pushrod and threw a lifter out of it's bore you will lose all oil pressure. Pull both valve covers and make sure all the pushrods and lifters are in place.
I had that happen to me, I think I torqued down the rocker shaft too quickly on a filled lifter. Not like I was winging it up to high RPM or anything, heck I was leaving the doctor's office with my daughter when I lost oil pressure.
I also had a very small chunk of casting flash break off in a 360 and get caught up in the oil pump, locked it up and snapped the end off the drive. Not even sure how it made it past the pickup screen.
