Bummer. But hopefully it's not something that will be terminal.
Can the crank turn a little bit in either direction, but then hits a hard stop? Or it is locked totally solid, can't budge it at all?
If it's the first, then you would suspect something having to do with the valvetrain or head, mechanical interference of some kind.
If it's the second, then either the pistons are rusted in the bores or it's hydrolocked.
Hydrolock situation can't just happen out of the blue, there has to be a way for liquid to get in. Is the coolant level still all the way up or did some disappear? Intake and exhaust systems are installed still, right? No way rainwater could get in?
"Broken cam syndrome" is one of the oldest and most ignorant myths about D24 engines that you may have heard. The legend (created by folks who don't understand) is that the camshafts just shatter for no reason, spontaneously. The truth is that they break AFTER the timing slips, either failed timing belt or spun crankshaft gear (often times due to botched installation of a timing belt just a little while earlier, by the very same person who now claims the camshaft just exploded out of the blue!). So I wouldn't go looking for a broken cam -- it simply does not happen, except when it's a result of another failure somewhere else.
But you do want to check the condition of the timing belts, as you say. Watch the belt as you try (gently) to turn the crankshaft. Is the belt moving at all? Slack area moving from one side to the other?
If the timing belt doesn't get you anywhere, I would remove all the injectors or glow plugs and see if that makes a difference. If no results from that step, then you are probably looking at some serious engine work to deal with whatever has happened, unfortunately.
Let us know what you find.
