How to save hardened fondant?
I often soften Fondant by microwaving it for between 5-20 seconds. This does work on fondant that hasn't been left for more than 2 months. Once fondant has been left for this long, albeit in an airtight container, it isn't likely to be salvageable.

Most manufacturers of fondant quote 2 months as the lifetime of fondant stored in an airtight container.