Some services, like texting and a few internet messaging apps, were reported to be working, but not consistently.

Mr. Galarza said he was optimistic that his family on the island, including his father and two older sisters, were doing well, in part because they had all hunkered down at his father’s sturdy cement house. He also noted that Puerto Ricans were old hands when it came to surviving devastating storms.

Just two weeks ago, Hurricane Irma sideswiped the island and left nearly 70 percent of households without power. Spared the worst of that storm, Puerto Rico soon became an impromptu hub for disaster relief, where residents of devastated islands fled to for shelter and supplies. Now, many of those who had provided help will surely be in need of their own.

Relief efforts were quickly organized despite the communication challenges. Mayor Bill de Blasio of New York City offered help to Puerto Rico in a news conference Thursday, saying that, “New York City stands with you and we will be there to help.”

“There are 700,000 New Yorkers who are of Puerto Rican descent and feel a direct, powerful tie to their homeland, and they are feeling this crisis,” he said.

Those sentiments were shared by Puerto Ricans in other cities. It was a feeling of “impotence,” said Eliezer Vélez, 44, of Atlanta.

Mr. Vélez, who works for the Latin American Association in Atlanta, said that he was hoping to get in touch with his mother, two brothers and a number of uncles and cousins. He said a sister who lives on the island was able to send him a message through WhatsApp on Thursday morning; she relayed that everyone was O.K.

“We’re praying for them and hoping for the best,” Mr. Vélez said. “It’s really sad that you’re here, but your mind and your heart are on the island. We are here, but we belong there. I cannot describe the frustration that I’m not there.”

Maricarmen Romero-Vazmina, a resident of Sarasota, Fla., said she was “freaking out, because I was able to talk with my mom five minutes yesterday and have not been able to communicate again.”

Ms. Romero-Vazmina grew up in Guaynabo, P.R., and works in Florida as a court interpreter and mediator. “We here are nervous, terrified by the news and the photos, and unable to do much for our families and the thousands that have no roof.”