Hi 
I've no idea why it happened. I don't think anyone else has reported
anything similar yet. Generally it's worth keeping back-ups whatever
program you are using.
I tend to add a "version number" to the end of files and occasionally
increase that number so that i can always go back to an earlier
version (just in case i stuff up or need something from an earlier
version). I try to keep it simple so i get file-names such as
Diwali-Poster-2013-v1
Diwali-Poster-2013-v2
Also i tend to keep artwork, logos, and old versions in a sub-folder
near the document jic everything goes horribly wrong. Occasionally i
go through and just delete all those sub-folders (sometimes quickly
checking through them first jic there is something interesting lurking
away in there (but my OS makes that really easy))
Tim (Kracked Press) idea of doing such frequent saves and back-ups
doesn't suit me at all! If i set the option it's for around 20-30
mins or even longer but usually i try to switch that off and just
remember to do my own frequent saves. We are all different though and
what suits me doesn't suit everyone! Diversity breeds serendipity.
Regards from
Tom 