imessage disappeared before being read

Hi,

Hope someone can help as I'm so confused! Last night I received an imessage which as usual appeared on the lock screen as a notification. I was just about to go to sleep so didn't read it and thought I'd leave it till this morning. When I woke up at 4am it was still showing as a notification on the lock screen but when I woke at 6am the notificiation had disappeared from the lock screen and when I opened messages there was no record of the reply ever coming through. I've had to ask the person to send the message again but I'm so confused. Has this happened to anyone before?

Thanks