"It was a stubborn fire," Severns said. The two-alarm blaze took more than an hour to extinguish and closed Route 38 East for several hours.
The fire appears accidental, officials said.
About 35 years ago, the Photoiou family bought the diner, formerly the Maple Shade Diner, with Maria and George Romanos.
Photios Photoiou had come from Cyprus. He loved America, and his wife followed him to the United States after he had been here a few years, she said.
The diner was a business where their three children, now in their 20s, have worked, and where some employees have stayed with them since the 1970s.
"I think everyone's in a state of shock," said Bernie DeCrosca, who has been a waitress for more than three decades and had been looking forward to the Christmas party.
"It's like family here," DeCrosca said. "They were the easiest people to work for."
Loyal customers streamed through the parking lot Monday. Some were surprised; others came to offer support.
"It's a meeting place. It's a place where comrades and friends come together," said Don D'Amato, 75, of Mount Laurel. He had come for breakfast mostly for 25 years. He liked the French toast on thick bread and triple dipped in egg. "The owners were hard working and friendly."
The counter crowd was an eclectic group, said D'Amato, a real estate broker and appraiser. It included an aerospace engineer and a bricklayer. Conversations ranged from work to girls to movies.
"It's like Cheers without the alcohol," D'Amato said. "It's a place where everyone knows your name."
Niki Photoiou said her husband had not felt well lately. He had a kidney transplant several years ago and initially was alright.
She does not know how they will get through this crisis, she said.
On the diner's Facebook page, a post read, in part, "Words cannot express the amount of gratitude we feel toward you & your years of patronage & loyalty. We have shared many good times & memories & we hope to be back soon."
Contact Barbara Boyer at 856-779-3838, firstname.lastname@example.org, or follow on Twitter @BBBoyer.