No, it isn’t necessary to keep an opened jar of Nutella in the fridge. Room temperature is ideal to store Nutella. It contains a high amount of fat from palm oil, hazelnut, and other ingredients. These fats will solidify at refrigerator temperature, harden the product and make it less spreadable. Once you have a Nutella stored in the fridge, you should finish it quickly after you take it out of the fridge. Otherwise, it can go bad faster. To soften it you can put the jar out of the fridge and keep at room temperature. But, this will take time. Try to scoop out the spread and place it into a microwave-safe bowl, and microwave for 15 seconds. Be careful, too much heat can separate the oil in Nutella and make it more liquid.