If there were a serious problem with the USD, first, because of it's seigneurage position in the world, it would imply a 'giant crisis'. It would be bad.
People would switch to other currencies like Euros, Pounds, Canadian / Australian Dollars and Swiss Franks, RMB and Yen because they are fungible and transactable.
Gold and other commodities would increase in value, but only as stores of value, they're not good for transactions.