Thanks for your replies.
To add some more information. This is the way I worked.
I prepared the backing as usual using Halion and Sampletank 3. They are still fine. I added the vocal lines as normal audio and created a folder for all the songs for the choir. In the main folder I created sub folders for each song where I saved all the individual MP3s that I transferred to the the project. I still have those MP3s but if I start again I will have to do all the editing again which will take days.
When I saved the project, simply saved ‘save file as…’
I have tried another song with midi and audio and have no problem.
I have an external hard drive which I have used as a backup. Maybe the project went there! The trouble is that drive now shows a message “The file or directory is corrupted and unreadable”. I hope to find someone who can deal with that problem so I can check there.