A day after signing his first long-term contract as an NBA head coach, Lawrence Frank wasn’t in a mood to celebrate.

Between the joy of extending his childhood dream and the news conference to announce it yesterday, Frank had to watch his Nets play a horrid game in a 100-77 season-opening loss to Shaquille O’Neal and the Miami Heat.

It was the franchise’s worst opening night loss since joining the NBA in 1976.Talk about letdowns just hours after signing a four-year, $10 million contract extension.

“You can search for reasons and I take responsibility, but we just played out of character,” Frank said. “We didn’t have much of a presence out there.”

That may not be too surprising this season. Frank has re-signed to coach a team that has been transformed from a leading contender in the Eastern Conference to one that will be lucky to make the playoffs.

There were three new starters – guards Jacque Vaughn and Ron Mercer and forward Eric Williams – in the opening night lineup, and six new players on the 12-man active roster on Wednesday night.

“I think that there is nobody that knows more about the game than Lawrence does, nobody prepares better and no one has a passion for this game any more than Lawrence,” Nets President Rod Thorn said.

For the most part, Frank negotiated his own contract, saving a 4% agent’s fee. He spoke with fellow NBA coaches for advice and got some legal help in fashioning the deal.

Frank, still boyish-looking at 34,took over as coach of the Nets in January after Byron Scott was fired. He won his first 13 games – a record start for a coach of a major professional team in North America – and posted a 25-15 record in leading the Nets to the conference semifinals.

The shake-up came after the season. After Martin was traded to Denver in a salary cap move, the Nets shipped Kerry Kittles to the L.A. Clippers and waived veteran backup guard Lucious Harris. Backup forward Rodney Rogers opted out of his contract after the season.

“Although some of the names on the backs of the jerseys have changed in recent months, the front still says ‘Nets’ and we will continue to give the fans the only style we know how to play, the unselfish, teammate-first brand of basketball that characterizes the New Jersey Nets,” Frank said.

A native of Teaneck, Frank began his coaching career at Marquette. He spent three seasons as an assistant with the Vancouver Grizzlies before joining the Nets’ staff.

“We are very optimistic about the future, and the future being today, not beyond today,” said Frank, who spent 3 1/2 years as an assistant with the Nets before being promoted. “We know we have a long ways to go.”