Renewed analyst worries about the ad sales outlook at Yahoo! triggered a sharp fall in its stock price, which fell to its lowest level in two years. Yahoo! traded at less than $39 before a Friday rebound. The stock opened the year last January at $250. A Morgan Stanley Dean Witter & Co. analyst predicted a 30% chance Yahoo! will miss earnings forecasts in the next three quarters. Yahoo! has said it is taking steps to broaden its revenue sources beyond advertising. In trading Friday, Yahoo! closed at $40.88, off 20.24% for the week.