Best case scenario. No country will do business with the US, forcing the US to become economically and industrially self sufficient again. Tremendous internal strife as those on the dole can not be afforded a free income any longer. US ends the Fed, outlaws fractional reserve banking, and dissolves any acknowledgement of debt to the Fed.
Worst case scenario. China and lender countries demand payment in land, and will take it by force if necessary. Ground invasion of the US sparking a third world war.
"Use more gun." -The Engineer- (Team Fortress 2)