I concur with Ellen. Unfortunately, the compatibility between the 2007 and 2010 versions of Microsoft Project is not flawless. I have seen issues where a 2007 file was opened in 2010 and tasks disappeared… even though one might hope that Microsoft Project 2010 wouldn’t have a problem opening 2007 files.
If you continue passing project files back and forth between versions, then your issues may start to compound until the file is completely unreadable.
To remove the data corruption, you can try opening the file with Microsoft Project (in whichever version shows the most accurate schedule), saving it in XML format, then opening the XML file and saving it back into MPP format. This is an old trick that sometimes works on corrupted project schedules.