Federal Reserve Notes vs Treasury Notes

It sounds like you're asking what would happen if the national debt were to be paid off if the Fed simply printed the money. It would not be the end of the world, but it would be the end of the U.S. dollar as a means of exchange, that is, the dollar would no longer function as money. The dollar It would find more use as wall paper than as a means of indirect exchange.

Holders of the $11 trillion treasury debt would panic upon learning about the Fed's intention to pay off the debt by printing money.  Everyone would want to sell their treasury securities immediately. Treasury yields would skyrocket, bond prices would plunge. The early recipients of the Fed's pay out might escape (perhaps by only a few hours) by using the newly printed Federal reserve notes (demand deposits) to buy other assets, like gold.  Later recipients would get crushed because, although they will be paid if full, the value of the newly printed money will approach zero, i.e., they won't be able to purchase much of anything with it.

Look at what happed to Zimbabwe. $1 billion might buy one egg.  As Americans, any cash savings would become worthless.  it would be the end of the world as we know it because money is essential if goods are to be exchanged.  With the in ability to exchange goods, social unrest would erupt. The government would impose price controls, shortages would result, marshal law declared to "restore order", a tyrannical government would emerge.

Other than that, nothing much would happen.