D6C Engine Locked When Attempting To Crank

It is a cold one up here in Canada this morning (10f)& when we went to start our D6C, there is plenty of power getting to the starter, but it just wont turn over. The starter really jars the engine & you can really feel it & it just moves the fan a bit but will not turn. It seems like the starter is fine, but there is something preventing it from turning over. Has anyone had something like this happen when it is really cold out? The amp meter shows full draw when I hit the starter & the drop with the glow plugs look like usual, so I don't think it is electrical.

I have started this machine in weather like this lots of times, when it is even colder out & it always started excellent.

Any ideas?

D6C Engine Locked When Attempting To Crank