Push notifications not being received

I’ve been using push notifications to my mobile on change for a couple of years, but they have suddenly stopped working. Looking in the logs, they are trying to fire, but there is the following error message (I’ve removed the device SID from the log message). Has anyone else seen this?

Message: cannot send the apple push notification

Attempts: 1

Cause

0: {“error”:{“message”:“apn write timeout”},“device”:

I should have said that when I use the “test notification” option in the app, this shows a messsge “Success”, but no notification is received. My mobile number is still configured to receive the notifications, so this isn’t the problem.