People across the world have been reporting problems with WhatsApp this morning.
The mobile messaging service seemingly fell offline for around 45 minutes - although it hasn't put out a statement explaining what happened.
Many users were faced with a spinning buffer wheel when trying to connect, with some still experiencing intermittent issues.
No reason has been provided for the outage but it is understood Facebook - which owns the app - is investigating.
The smartphone app uses symbols to inform users of how the transmission of their message is doing.
Worldwide, users were being shown the clock symbol after attempting to send a message, meaning that the message has not been sent to WhatsApp's servers.
The website for the messaging app also appears to be down in what may be a domino effect.
If a significant problem has affected WhatsApp's global messaging servers, it is possible the flow of traffic to the website from concerned users may have caused the outage.
The outage may be an embarrassment for Facebook, which has this week announced record revenues from advertising.
The Down Detector website's heat map showed reports were especially common in Europe, although complaints on social media came from people located worldwide.