It was 2.4 which I realise is quite old. However I have resolved the issue. In the "View" tab, in the "Mode" menu someone had changed from "Default" to "Setup". Setup only shows the preview pane presumably for setting up a service, and there is also a "live" option for running the service. I am not sure why you wouldn't just want live and preview available at all times, particularly when your worship leaders add unexpected songs during the service !