The Nasdaq stock market suffered a massive computer failure yesterday triggered by a huge sell-off of technology stocks, leaving thousands of investors unable to sell shares as prices plummeted. Traders and institutional investors said the breakdown, which lasted more than an hour, was the most serious in a series of computer failures that have dogged Nasdaq for months as the daily volume of shares traded has soared, although yesterday's volume was well short of a record. Brandon C. Becker, director of the Securities and Exchange Commission's division of market regulation, said the SEC will examine what happened. Nasdaq has no trading floor, but is made up of independent dealer firms linked together by telephones and various computer systems. Nasdaq shut down its two major computerized trading systems at 9:56 a.m. after discovering that its systems were so overloaded that most computer terminals used by dealers were displaying stock prices that were 12 minutes out of date.