Okay, I've searched around, and found that Project 2007 Service Pack 1 is NOTORIOUS for corrupting projects when saving them. Your project is probably gone for good if you don't have the latest Service Pack installed. You'll need to start from scratch, and make sure you update to Service Pack 3 or newer. If you already have it, then it probably saved the file inproperly (corrupted it) even so, and you'll need to contact Microsoft Support about the issue, and provide the corrupted Project to them as well.
Edit: also, try this:
Go to File --> Options --> Trust Center --> click on Trust Center Settings --> Select the option "Allow loading files with legacy or non-default file formats" and try opening the file.