I've been working my way through all the levels, collecting the Locate Hidden Realms along the way.

I've saved at the start and end of each level. I'm now ready to tackle Level 20. I wanted to use one of the secret levels I had previously collected to transfer a creature. Lo and behold, all the previous Located Hidden Realms are no longer in the map.

Help me get the secret realms back.

Thanks

I don't think that is possible. I'm not even sure how you managed to lose them.
Since you're at lvl20 anyway, what you can do is just launch the game directly into a secret level by adding '-level 100' to the end of your shortcut. When you transfer a creature, after that you should be able to launch lvl 20 and get the transferred creature.

The secret levels have numbers 100 to 106.
(One of them can't be unlocked through playing the game normally, as the hidden realm that was supposed to be found on lvl18 was forgotten.)