If it has come out of the blind roller itself you will have to remove the blind from the window first then extract the clip in black "module" that holds a spring which is supposed to attach to the string cord.
To remove the blind get your fingers behind the blind in each corner and pull hard at 90 degrees to the window ... it should suddenly pop out but does require a fair amount of force.
If you buy the spare part from velux it comes with a picture style set of diagrams to replace it.
You may be able to extract the little plastic module without removing the blind but probably will be very awkward. The module itself can be opened by unclipping the one side - it is effectively a single piece of plastic that folds in half and clips together.