file is not a valid freehand document

Hi

I need help! I am a Freehand 9 user using Window 200, 256MB Ram. At times i have a problem of not being able to open the freehand file that i save. The error code is "Could not convert the document because the file is not a valid freehand document ".
I have tried using Freehand MX to open but the same error code appear too.

Was the file corrupted ? How can i prevent this thing again?

Cheers
Bwlim