What was the economic result of demobilization in 1920 America on war supply industries and cancelled arms contracts?

Right after World War I, the US economy went into a bit of a recession.  This was largely because of the fact that the war had ended.  So the result, I'd say, was the recession.

As you mention, the end of the war led to a number of problems.  Factories that had been working at full speed producing weapons and such were suddenly put out of business, for example.  And this loss was not made up for by other kinds of business, especially not right away.  This is partly because foreign countries were no longer so able to buy things from the US.