Don't get your hopes up, but try to resurrect it. Refill with oil and try to start. If it doesn't want to turn over, fill the engine all the way (as in completely up into the valve cover) with oil and let it sit for a day. Drain the oil out to the proper fill level, pull the glow plugs out, and put a ratchet on the crank. Try to turn it over (clockwise only) and if it does, turn the engine several times to get all the oil out of the cylinders. Once it's rotating freely, try to start it again. If it seems to have been saved, run it carefully. If your hot oil pressure is noticeably lower than it was before, you have done damage to the main and rod bearings.
I'd get that spare engine prepped for surgery just to be safe.
