It sounds like it got corrupted then. I doubt you will be able to do anything to recover it… But at least you have the option to try to save the information from the file using Illustrator Recovery Toolbox. http://www.oemailrecovery.com/illustrator_recovery.html
Are you working off of a local hard drive or across a network to a file server? You might have some issues with your network if it is the latter.