It could be Windows failed while trying to rename the temporary file. This might result in a missing project file.

ido1957
With respect the quarantine area sounds unlikely.

So is a file disappearing. Extremely unlikely but nevertheless happened. Yes it's just a small possibility and I just thought I'd make the suggestion and you might want to check it (just like chkdsk suggestion you queried which was also a possibility). I will avoid any further suggestions as you might not like them either.

Ta

 I run the defaults settings in Sonar X3. The project was saved weeks before and also several times during the day I was working on it.

Default settings? so you never defined a folder saving your projects and you never set up CWP and per project audio files? Your project is where it was last saved at. Nothing will disappear, even with errors. The projects last save will not be harmed. 
 
Also, i saw you asked about a back up./ did you back up your project to another hard drive or CD or DVD? Look there to get your backup. Sonar is only as smart as the person using it. Do not assume you have back ups if you never backed it up.
 
CJ