But Mr. Turner and others who depend on the technology learned just how risky that strategy can be when the Dexcom follow feature suddenly went down without warning Friday night — right as people were relying on it to alert them to nighttime emergencies. Jake Leach, Dexcom’s chief technology officer, said late Sunday night that the outage occurred because the company’s servers unexpectedly became overloaded. He said that a “large portion” of Dexcom’s users were affected, but he did not know precisely how many or when the service would be restored.

“We have a large team that’s been working around the clock on this since Saturday,” Mr. Leach added.

It was not the first time the service went dark: Dexcom experienced a similar outage less than a year ago, on Dec. 31, which it resolved within a day.

But this time many users were furious because Dexcom did not announce there was an outage until about 8 a.m. Pacific time Saturday, which is 11 a.m. on the East Coast, when it posted a brief notice on its Facebook page. The post received thousands of comments, most of them from loyal users who were upset that the company did not notify them sooner.

“Dexcom, please, please send a notification of some kind when your products are not working,” one distressed parent wrote. “We are fully capable of monitoring our son when his devices are down, and are certainly used to sleepless nights checking blood sugar. I understand that software fails at times, but silence to parents dealing with life and death situations is completely unacceptable.”

Heath Smith, whose 11-year-old son Cole has Type 1 diabetes, assumed when he woke up on Saturday morning without getting any alerts that his son had slept through the night with steady glucose levels. But he was stunned to find a blank screen when he opened his Dexcom app to check on his son. Mr. Smith ran upstairs and shook his son awake. It turned out that his glucose levels had plummeted to a severely low level of around 40 milligrams per deciliter and hovered there throughout the night.

“We’ve learned to depend on getting an alert from the Dexcom and we never got it,” said Mr. Smith, who lives in Lawton, Okla., and works for a medical technology company. “We used to set our phones for every two hours throughout the night to go and check on him, but we haven’t done that in years because of Dexcom.”