If US bond had no buyer, government can not pay interests on its existing bond. Bond yield will skyrocket and bond market/stock market/economy will tank.
Government will have no other way to finance its spending. It will either go bankrupt by default or go hyperinflation.
China's purchase of US bond is a tempary strategy in order to provide its labor force job stability by supporting US spending, in hope it could transform its dependency from export to domestic spending. They hoped there will be enough time but I bet there will not be.
Worldwide collapse followed with new Brettonwood system is one of the likely scenario.
WW3 is another scenario.
