U.S. stocks fell sharply in heavy trading on Monday and the S&P 500 and the Dow had their worst day since October after a collapse in Greek bailout talks intensified fears that the country could be the first to exit the euro zone. "There is no mechanism to be ejected from the European Union. This has never happened before," said Brian Battle, director of trading at Performance Trust Capital Partners in Chicago. "When you don't know what could happen you sell. You get on the sidelines."