Many WhatsApp users across the globe cannot send or receive messages using the Facebook-owned application, apparently because of a server problem.
The app with a billion people in over 180 countries doesn't initially look broken, with chats and contacts able to load.
But once a user is into a chat, the app just shows a "connecting" message that never resolves itself, and so new messages can't be sent or received.
That also means there's no easy fix to the problem since it appears to be on WhatsApp's side.
Shutting and opening the app, for instance, just resets the connecting message but won't stop it showing.
The website down detector showed a huge spike in problems with the app, suggesting almost everyone is unable to get online.
The problems are focused in western Europe and Asia, but were reported across the world.
There are relatively few reports of problems from the Americas, but that might simply be a result of the fact that timezones mean that many people there are currently asleep.
WhatsApp Web, which uses the phone's connection, is also broken with the App users expressed
frustration
across the world under the hash tag #Whatsappdown.
Reacting to the hitch, one user said he was reminded how to send an actual text,.
Another said: "A billion people look up from their phones for the first time in seven years."
Another person said: "feeling irritated".
WhatsApp said it was working on a solution.