Small Engines (Lawn Mowers, etc.): B&S engine wont turn, starter cord, valve spring

Question
I have a B&S 128600 series 7hp OHV engine on my lawn mower.  When I try to pull the starter cord, the engine won't turn, like it's seized.  If I remove the spark plug, it turns freely.  What might the problem be?

I have a hunch that one of the valve springs is broken but I can't figure out how to get the muffler off to access the valve spring cover to inspect the springs.

Answer
Are you holding the lever on your handle when you turn your engine over.

have you reinstalled the spark plug and re tried turning your engine over?  I am thinking that u may have had a temporary case of hydro lock...(where the cylinder gets fluid between the cylinder and the head)

A broken valve spring would not cause the symptom you describe