there is problem in your filesystem so that either the bits of file is unreadable, or the file pointer points to invalid location and lost.
If you haven't tried to find an automatically backup-ed copy from the "Previous versions", I highly recommand you to try finding it now. Typically shadow copy feature will only keeps verions 2-3 days depending to the free space left on your filesystem.
take a backup of original delete it and copy the code in new and open.
Let me know if u r still facing same problem.