Lower mileage than that of other that have failed. remove your spark plugs, see if any are smashed. also try to shine a light into the cylinders.
with the plugs out, you should be able to crank the engine aroubd with the starter... should go pretty slow though.
if you don't see any visible signs as mentioned above, it may be time to pull some bearings on the bottom end. At the same time though, check for metal chunks of the piston in the oil pan etc.
I wouldnt doubt that if the seat dropped just right it could seize the engine though.