Seized 12 hp Briggs and Stratton Engine

Briggs and Stratton 12 hp engine.
Model:  283707   Type:  0148-01
 
Until now the engine runs great.  I finished mowing for the season and put the lawn tractor away.  I parked the tractor and turned the engine off by turning off the fuel shutoff. (this is how I always put the tractor away after each use).  A little while later in the day I decided to change the oil.  I wanted to warm the engine a little and when attempting to start the engine it would not turnover.  I suspected the battery since the temperature was in the upper 30’s by this time.  I gave the battery a charge and had no luck.  I realized the engine was seized.  I tried to rotate it by hand but it would not budge.  This issue has never happened before and after reading so may possibilities I wanted to present this issue here for help.
I did remove the spark plug to try to relieve pressure if that was the cause.  The engine remains seized. 
Thank you.

Thank you for your prompt reply.  Everything is disconnected from the crank except the starter.  ( do you feel I should remove that ?)   I purchased some Marvel Mystery Oil because I read something online about it.  I have not used it yet.  Do you think I should use that instead of the 10W30 you recommend?   The engine is installed on a lawn tractor.  In order for get the spark plug opening into the orientation you suggest I would need to remove the engine from the tractor.  Do you feel I can get enough oil into the cylinder by jacking up the front of the tractor to give it a steep slope?
Thanks again for your help with this.

I realized my initial post did not include an important point.  Prior to the engine seizing the oil was at a full level and is never low at anytime of use.  I am at a loss as to why this engine problem occurred and how to prevent it in the future.
I’m going to attempt to fix this today. (I will let the oil sit for 24 hours)  I’ll let you know how it goes. 

Excellent!  
 
Just an FYI.  For engines not going to be run , such as generators (down here in Florida) for possibly quite awhile, I do some winterizing.
 
Drain all fuel from the tank (no fuel can't go bad), run the engine / carb dry.  Squirt maybe some WD 40 down the carb fuel inlet.  
 
Remove the spark plug, put a couple of teaspoons of oil / Seafoam, etc and rotate the engine over slowly(distributing the oil thru out the top end). 
 
Important: turn the engine over slowly and get to the compression stroke.  Stop there, re-install the spark plug, cap, etc.  The idea is to keep moisture out of the combustion chamber and rusting the cylinder walls.  The piston at TDC, valves closed does that.  
 
I also like to spray down the machine with some sort of protectorant to help protect the exterior.
 
I also, for my generator customers, DO NOT start the machine every month (especially with E fuel) and turn off.  The fuel evaporates, varnish builds and builds more every time the machine is run, till the carbs clogged..
 
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postimg.cc/VrtwX4pV
 

Ouch on that starter!    
 
Your welcome and take care..
 
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