HOLLYWOOD — For forty years the word HAROLD’S hung dark over Cahuenga Boulevard, a skeleton of gray glass tubing and rusting brackets that most people below had long stopped seeing at all.

On Thursday night, a little after nine, it turned back on — first a stutter of pink at the H, then the whole word warming to a steady rose against the dark, while perhaps sixty people on the sidewalk did the thing people do at fireworks, which is to say “ohh” before they can help it.

The sign has topped the two-story building since 1946, when Harold Ruiz opened a men’s clothier that outfitted, by family legend, at least three grooms every Saturday through the postwar boom. The store closed in 1985. The neon went dark not long after, when a transformer failed and no one could justify the repair.

“My grandfather couldn’t bear to scrap it, so he did the next thing, which was hide it,” said Diane Harold-Ruiz, 58, who now owns the building and leases the ground floor to a coffee roaster. The original transformer — a heavy, humming box the size of a toaster oven — spent four decades wrapped in a moving blanket on a shelf in her uncle’s garage in Van Nuys, labeled in grease pencil: DO NOT THROW OUT.

That box turned out to be the whole project’s keystone.

“You can rebuild the glass all day. The hard part is finding someone who kept the guts,” said Reuben Ostrander, 47, the neon craftsman Harold-Ruiz hired to restore the sign. He spent three weeks matching the exact color — a rose-gold he called “impossible to buy and easy to get wrong” — heating and bending tubes by hand in a Frogtown workshop that smelled, he said, “like a lightning storm you can stand inside.”

Neon once ran the length of these boulevards, spelling out cleaners and cafeterias and camera shops in a low electric hum that was, for decades, simply what nighttime in Hollywood looked like. Much of it came down in the lean years, scrapped or left to rot when the current went off. Preservationists have spent the past two decades coaxing pieces of it back, sign by sign, with the patience of people restoring a cathedral one window at a time.

Harold-Ruiz did not set out to join a movement. She set out, she said, to answer a question her grandfather never quite let go of: whether the thing still worked. It cost more than she will admit on the record. She does not regret it.

At 9:04 the sign held steady, and Harold-Ruiz stood across Cahuenga with her phone down for once, just looking up. Her grandfather, she said, used to leave it burning all night, on the theory that a lit sign was a promise the store would open in the morning. For the first time since 1985, the promise is back on the meter.