NEW YORK, DEC. 9 -- A power failure disrupted trading of over-the-counter stocks for about three hours today, leaving brokerages across the country in the dark on vital trading information and with a backlog of orders to process.

However, many dealers said that business had been slow and that trading was not significantly affected by the shutdown of the National Association of Securities Dealers Automatic Quotation system.

The National Association of Securities Dealers said the power failure knocked out the main computer that runs the Nasdaq system from Trumbull, Conn., at 10:43 a.m.

The system started to come back on line by 12:05 p.m. through a backup computer facility in Rockville, Md., the association said. It said most service, including current price information and volume, was restored by 2:30 p.m.

The problem affected the data that the association reported to news media for inclusion in stock tables. For instance, the group was unable to transmit to The Associated Press the bid and asked prices for its lists of less actively traded issues, and the NASD said incomplete volume figures were reported for all OTC stocks.

The over-the-counter market is the busiest stock trading system in the country after the New York Stock Exchange. The OTC market primarily involves stocks of smaller companies, although shares in better-known companies, such as Apple Computer Inc., also are traded.

"This has been the longest disruption of service due to a power failure," said one NASD official. The system was out for 10 minutes in August and for two hours because of a hardware problem in October 1986, he said.

One over-the-counter trader at Shearson Lehman Bros Inc. said she noticed problems with the computerized system beginning around 9:45 a.m. "You couldn't get anything and you couldn't enter or receive anything," she said. "People sat back on their orders and waited."

Laura Grupe, an over-the-counter trader with Coughlin and Co. Inc. in Denver, said her firm also was faced with a small backlog of orders after power finally was restored. "It would have been a lot worse if we had been busier," she said.

William Howard, a vice president and manager of over-the-counter trading for Sutro & Co Inc. in San Francisco, also said the shutdown was not a serious problem. "It was more frustrating than anything else," he said.

"It didn't really knock us out of kilter in terms of doing business because things have been slow anyway," he said