Until just before 7 last Sunday evening it looked like tha members of the New York City Opera, now performing at the Kennedy Center, were set for one of their happiest days.

Their matinee "Merry Widow" drew raves and their upcoming evening opera was "The Marriage of Figaro," one of their most celebrated productions.

That happiness ended abruptly - and a pall was cast over the company's whole 2-week engagement, when John Pintavalle, the orchestra's esteemed concertmaster returned from dinner and went to his locker. He found its door open, his combination lock opened, and his violin missing - apparently stolen. Now, six days later, there is still no trace of it. Park Police are operating on the assumption that it was stolen. Such a disappearance has never happened before at the Center and such events are rare anywhere.

The loss of the instrument on which he has based a career is particularly devastating for a string player. Says Pintavalle: "It's like a singer losing his voice. Each violin is unique. This one was particularly distinctive. And the plain fact is that it could never be replaced, regardless of how much money I had.

"And one of the lesser of my worries is that it is grossly under-insured, at $17,000." The 1757 Venetian instrument by Dominico Busan is worth roughly three times that amount, estimates Pintavalle. "This," adds Pintavalle, "is not exactly the same thing as losing an Oldsmobile, you know."

Pintavalle explains his dilemma: "A string player has a different relationship to his instrument from, say, a pianist. When a pianist tours he normally plays the local piano, wherever he is. A violinist or a cellist plays his own instrument. I bought this violin in 1964 and I guess I had played it an average of nine hours a day ever since.

"Of course, wind players travel with instruments too," acknowledges Pintavalle. But there are major differences - one being that winds don't cost what strings do. "The problem is that the best strings were all made in the 18th century, and the supply is dwindling, with inceasing rapidity, in fact."

But more important than that is the way the string player comes to relate physically to an instrument. "Mine wasn't even the flashiest in the Opera orchestra," says Pintavalle. "For heaven's sake there are two Strads and an Amati. But it was the right one for me. But it was the right one for me. It fit me in size and I love the dark sound of it. Its tone appeals to me because that's the way a violin ought to sound, so far as my ear is concerned.

"I remember the day I spotted it at Wurlitzer's in New York," a distinguished firm no longer in the business.

"I remember Mr. Wurlitzer let me borrow it for the evening, even though I was only a student at Juilliard. A friend who had a Stradivarius joined me at the Rogers Auditorium of the Metropolitan Museum, and we took turns playing the instrument to the empty hall. One would play on stage and the other would go out in front to listen. I decided I liked mine better, and I've never had reason to change my mind."

Just how and when the violin came to disappear last Sunday is unclear.

"All I know for sure is that I put it in the locker about two hours before I came back and found it missing. My lockermate's trumpet was still there. And there was no sign of tampering with the lock. It's in the musician's lounge and hundreds of people of all kinds must wander through there every hour.

"One thing I believe is that it wasn't taken by anyone in the company. For heaven's sake, even the people who don't like me have come up to say they're sorry."

For the present, Pintavalle, who is 46, is playing what he describes as "a very minor French" violin he has used for a backup.

Meanwhile, he is waiting. He has posted notices at the Center offering "a generous reward." He says, "I still think it might turn up somewhere down there. And if it doesn't my best hope is that it's in a pawn shop somewhere, instead of headed abroad to a place where the demand for strings is even greater than here - and the prices are higher."