How are you opening the file?
If you are clicking on the list of the last n used AND you've changed the
folder in which the file is kept, it won't find it

I would suggest you try to open the file by double clicking on the file name
from within Windows Explorer.  If it opens OK you have a pathing problem.
If it still doesn't open the problem is with the file itself which has
become corrupt in some way.  This may be a virus so it would be a good idea
to update the virus signatures and then to scan for viruses.

If this doesn't work come back and we'll have a re-think.

Regards.

Bill Ridgeway
Computer Solutions