Notification ringtone does not work even though it is on full

we have a Samsung galaxy tab running android 4.04. This morning the notification ringtone did not work for any app. When I check the setting for volume, notification ringtone volume is on full BUT the icon on the left is displayed like it's on mute with a line through it. Furthermore there is the same icon (mute notification) next to the battery at the bottom right of the screen. I have tried changing this ringtone but nothing plays. I believe the tab thinks that the notification ringtone has been set to mute when it hasn't! Any ideas how to restore?

Thanks