What would happen to the stock market if the U.S. dollar lost the reserve currency title to another country?

Okay so I want to preface this by saying I know it would be bad for the U.S. but what would actually happen? Would everything just immediately tank the next day and mass panic would ensue or something more slower and gradual? Does anyone know what happened when the pound sterling lost its dominance to the U.S dollar? Sorry for so many questions but genuinely curious on this topic.