Gravely ZT1844, has been running fine, now stopped 30 seconds into the latest cutting session. Single pot, vert shaft Intek 18hp. Almost felt like a ran over something that got tangled in the belts and locked it up...it didnt sputter to a stop, it just stopped. All belts removed, crank will absolutely NOT move, even with the plug out with a breaker bar and socket on the top bolt. I pulled the head and the piston is not stuck in the bore. (20 year old engine, piston has a little slap in it...). Is there precedence for the bottom bearing to seize on the engines? Looks to be a small amount of oil on the bottom seal...Help?

I have the same engine which dropped a valve seat and destroyed the head and piston. The cylinder was fine so  replaced those parts 3 years ago.   Of course, that's not your issue..
 
There is a large/weird counter balancer on the crank that may be the issue.  In any event, it seems you'll have to remove the sump to inspect.  And as long as you had oil in the engine/air flow under the cover, the crank didn't seize to the rod...
 
A look at my balancer:
 
 
 

You can see the aluminum balancer hanging (as the engine is not in the normal position) from the crank.