A family of Microsoft relational database management systems designed for ease of use.
JPBehr,
It's possible the one file is corrupt. I'm assuming these files are not part of a linked structure (i.e. master/subprojects).
Normally saving to XML and then re-opening the XML file in Project is a way to clear corruption but it doesn't always work. If you are still able to open the file in Project, you could try methods 1 and 3 in the following Wiki article:
Hope this helps.
John