WhatsApp has fallen for thousands of people across the UK, Europe and Asia.
Users began to worry about not being able to send and receive messages Sunday morning.
More than 500 million people use the app daily, which is owned by Facebook.
According to Down Detector, London is the worst- affected area in Britain, with Paris, Belgium, Germany and the Netherlands least affected.
Areas of Asia and the Middle East, including Dubai, India and Malaysia are also suffering problems.