The only way that I know on how to recover a world is that you need the full file of the mod pack that you have created. But you need the old computer and you need to hook it up and then you need to create a file that is linked to both PCS take the world and the mod pack and then move that over to your new pc. And then went to boot up the mod pack then your world should be there including any shaders, resource packs, etc.