I would remove the belts, valve covers and sparkplugs, then try to rotate the crank in both directions with a breaker bar, watch the valve train. If still locked up and nothing obvious in the valvetrain, remove and cut open the
oil filter looking for metal particles, if none, then drain the
oil and remove the pan. Check the pan for any chunks of metal too big to foul the filter, drop the
oil pump and shaft and try the breaker bar again. If it still will not turn, I would suspect pistons, rings or valvetrain, if you didn't lose oil pressure or hear knocking from the big end or piston slap before shutting down the last time, I doubt its bearings. You could pull the main caps to check those but if its locked up, you probably can't pull all the rod caps. Thats about all I can think of to check with out major disassembly or pulling the engine. Good luck.