A Google search on repair .mpp project file will reveal several options, from downloading tools to try and fix the problem, to saving the file in another format e.g. .xml to try and recover the data. It appears the problem most commonly arises with shared data systems, which makes sense as you would tend to share project data among several users.
Somewhere along the line, the file has been opened somewhere and failed to shut down properly, possibly due to a sudden loss of connection. If all else fails, you may be able to recover some of the data or manually edit the file with an hex editor.