Help! MS Word for Mac 2008 advice - doesn't recognise it's own pw protected file!

Hi - i've got a real problem.  I've been writing my speech for my upcoming wedding in MS Word for Mac 2008 and decided to password protect it when I saved it, in case curiosity got the better of a certain someone.

After successfully creating the document with all my thoughts in, applying a password and saving it, I have quit Word.  Upon trying to launch the file to edit the document, it doesn't seem to recognise the file (it's own file!) - it doesn't prompt me to enter my password, it just comes up with a 'convert file' box which appears to be asking me to select the source format of the file - I've tried selecting 'Word' as well as a few other options and in each case it either errors out or opens and appears as gobbledygook. I've tried opening it from Finder and from within Word with the same result.

I really don't want to lose all my hard work - I was pretty happy with what I'd created and I don't think it will be as good if I start again from scratch. I have tried googling the problem and found nothing which matches my issue.

Does anyone have any idea what might be the issue? It's makes no sense to me.

Thanks!