Helena L

The project was completely created in the new version. I appreciate your comment though as I do have a few beta projects that may be resurrected at some point!

That’s really great to hear. Nothing’s worse than losing a day’s hard work. 

That's good to hear - If the file was form a beta version I would recommend remaking the tables from scratch in the release build

What about losing a week's hard work?
 
This should make no difference.

Thank you Paul. Affinity hadn't properly quit (probably why I couldn't open it) but I managed to force it to eventually and restarted. It's all back; Affinity Publisher, and my file! I'm so amazed and very relieved. Thank you to everyone for all your help. H

Thank you for these helpful answers. I didn't realise about Time Machine, and sure enough when I checked just now, there was the file. I haven't restarted since it happened 'cos that once meant I lost stuff but I'll go ahead and try it now. I did wonder if I should have deleted the Beta version of Affinity which is still in my applications folder. 

So Affinity will not run up even after restarting the computer ?

By default, I think OS X Yosemite should also take local snapshots of your files:
https://support.apple.com/en-us/HT204015
So there might be a chance that your file isn’t completely lost. I would try to open the folder you had saved your document to and invoke Time Machine. Maybe you can find an earlier version of your file.

Did you do a search for the filename using Spotlight?
Do you have Time Machine enabled, so you can go back to when the file was still in the folder?