Super Nintendo cartridges may survive longer than other game hardware, but the batteries in them only have a life expectancy of about 10 years.
Those batteries are responsible for retaining save data, and once they're gone, so are your game files. But if yours is already dead and you want to be able to save in the future, this guide on Motherboard can teach you how to replace the watch battery inside any cartridge.
Some SNES cart batteries can last much longer than 10 years, though, so don't assume yours is fried. Test it first. As soon as you remove the battery, you'll lose your data whether or not it has already disappeared.
[Kotaku]
