This has happened to me. It is because an error in the planet files, like the game is looking for something it can't find (like a texture, or a body's parent). What to do:
Go to your planet files and remove them all, and reopen the game. Next, place them one by one. When you find yourself in the same situation as before, go to the last one you copied, thats the one who's causing the problem. Open it and fix it. Then you can simply put your planet-pack where it was. If you had a saving, load it so you dont lose your progress.