In Grunt, data from an external data source such as Excel can be overwritten by changing the value directly in the cell. When done, the underlying data does not disappear but is temporarily replaced by whatever new value you entered. You can recover the underlying value by opening the data editor (press F3 or click the pencil icon to the right above the Rule stack), and looking for the overridden cell - it will have a yellow tag attached to it. Click the cell and press the Del key on your keyboard. The underlying value should now appear.