The Dow Jones industrial average fell 204.78, or 1.3 percent, to 15,666.44.
The exchange sent out an alert to traders at 12:20 p.m. EDT saying that trading was being halted until further notice because of problems with a quote dissemination system.
Stock indexes lost 2 percent and gave up nearly all of their gains for the year Wednesday. All 10 company groups that make up the Standard & Poor’s 500 index fell.
The Federal Reserve expressed more confidence in the U.S. economy even as Japan’s nuclear crisis raised worries around the globe. The Fed said Tuesday that the economic recovery is on “firmer footing” and the jobs market is “improving gradually.”