If you're using the Steam or GOG version, the Mystery Box rewards should be restored as soon as you start the game with the Steam/GOG client running. If you're using a DRM-free version, you can save and restore your Mystery box contents through the box (the button in the upper left corner of the box). In that case you've probably out of luck, because you would have had to save the box beforehand and retain the code that it gives.
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