When you say the font is 'missing', do you mean that you can't find it in the drop-down font menus at all, or just that it is marked as missing when you open the file and you can load it in using Find Font?
If you create a new document, can you select and use your problem font there, or does this only occur in specific documents?
Does it work if you put it in the document fonts folder?
What about the InDesign fonts folder?