Well, if your coolant was 2 liters low you shouldn't have done any major damage to the block even if it did freeze up. The block will only crack if there's no place for the expanding water to go - in your case it had a good bit of room, I'd guess. And as long as the remaining coolant was the proper mix for the temperatures, it shouldn't freeze anyway.
You did have the compression release open when you tried to turn it over, right? Makes a huge difference, of course. With extremely cold oil and full compression it would be extremely difficult to turn a diesel over by hand.
A warm garage would make working on it much more comfortable and would allow you (eventually, after thawing) to rule out the flywheel being iced up and frozen in place.
Perform the usual checks for oil in the coolant, coolant in the oil pan, etc. That will determine if you have a blown head gasket. If so, you'll want to get it opened up as soon as possible and clean out the cylinders that have gotten coolant in them, before any damage to rings, pistons and cylinder liners occurs. While you're at it, check the engine to see if any of the core plugs (sometimes called "freeze plugs") show signs of being shifted or pushed out.
Things may not be too bad - you just have to take it one step at a time and determine what is happening. It is a pain in the butt to have to do it during the dead of winter, though.
