But a power surge that accompanied the utility's resumption of service disabled Nasdaq's mainframe computers and seriously damaged the electrical system at the complex, making it impossible to use backup generators.

Nasdaq officials then switched to a backup computer system in Rockville, Md. By 12:05 P.M., service had been partly restored. Full service was available by 2:30, Enno Hobbing, the Nasdaq official, said.

Mr. Hobbing said Nasdaq would continue to operate the system out of Rockville until repairs had been made at the Trumbull facility. He did not know how much the repairs would cost or how long they would take. But he emphasized that the Rockville operation could handle the over-the-counter market, the nation's busiest stock trading system after the New York Stock Exchange.

Because of the breakdown, options trading on over-the-counter issues was halted for several hours at the Chicago Board Options Exchange, the American Stock Exchange, the Philadelphia Stock Exchange and the Pacific Stock Exchange.

Market makers in over-the-counter issues resorted to trading by telephone - the method used before the automatic system was instituted. Those interviewed refused to blame the Nasdaq system, which has been criticized for its inability to handle the huge number of customer orders during the recent stock market collapse.