There are lots of things that could trigger a collapse of the United States economy, but there are also lots of safety nets in place to prevent that from happening. The Federal Reserve's main job is to protect the economy of the United States and they enact policies based on science and research that stabilize the economy.
The Federal Reserve may not be enough to protect the economy from a major event in the United States however. Something as major as nuclear war, significant internal political upheaval, or a major natural disaster may provide enough instability that the economy could collapse.