My advise would be to go into explorer and make sure the file is still there
and still has the right extension and everything and you didn't make it
read-only, etc. Then double-click it in windows explorer and if you get this
message, it sounds to me like it is "toast".

Next thing I'd do is reboot my computer and try accessing the file again.

Finally, I would then go to a backup copy (you do make backups, right?). Or
if no backup, you may have to start over. Good Luck!