Have you tried doing "File Save As", give the file a new name. Then you can manually go an delete the original once you have saved as a different file.
The original may have become corrupted somehow (power out while open, reboot, BSOD etc).
You could also check the file properties, and make sure it is not set to read only.
Is the file on a local drive on your computer, or on another computer?
If it's on another computer, and you can't do a file save as then you probably don't have permissions on that computer, and saving the file to your own hard disk might work.
hth
Tanya