Hi,
It sounds like you may have been infected by a virus. One such virus (Ransom-ware) encrypts all your files and charges you to un-encrypt them. I would look into such a virus CryptoWall HERE(http://www.bleepingcomputer.com/viru...e-information). A number of my clients had a previous incarnation of this virus and excel had the same behavior you described.

Good Luck,
Gus