Scrivener says 'is not a project file' - Help

Hi all,
I’m a new user and have been heavily using this for the last month. Finding it great. However I spent all day writing today, and backed up several times. Unfortunately my back up files will not open. I tried double clicking and opening through Scrivener itself but it doesnt work. If I double click the file, nothing happens. If I try to open through Scrivener it says ‘The file is not a Scrivener file. Please select a valid Scrivener file’. Does anyone know what has gone wrong here or perhaps have a fix? Its really frustrating.
Thanks a lot

Do you have zipped backups?