What was wrong with my gerbil, and can I re-introduce her? [details in question!]?

[chocoholic_gerbil]

My two female gerbils have appeared to be okay. Yesterday, I didn't play with them, I just changed their food and water. Recently, they have been trying to 'hump' each other, they both seem to try, and the other one just runs away.
We came down this morning to find one of them laying there, very still. She was very cold, and had her eyes half shut. She couldn't seem to move her back legs, she was dragging herself around when I opened the cage, but only with her front legs.
We decided to move her into the spare cage we have for cleaning them out- the cage we have for them normally has multiple 'floors' and we were concerned she could drag herself up, and she could fall, as my one of my previous gerbils did. We were also worried that she could pass something on to the other gerbil, or whether they had fought, or whether the other gerbil would antagonize the ill one further, so we put the ill one in a cage with wood shavings, food, water, and a 'house' with bedding.
I was, in all honesty, expecting to return home to find her dead- my mum was going to take her to the vets if she hadn't improved by this afternoon, as she was working this morning. I came home, and my parents were out, but my ill gerbil seems much better!
She came over to see me when I approached the cage, and seems to have regained use of her back legs, so much so you can barely tell anything was wrong!! She seems to be warm again, not cold and clammy too...
I have two questions..
1) What was wrong with my gerbil?
2) Would it be okay to re-introduce the ill gerbil back into the cage with the other one, as she has been out of the cage for around 12 hours?

Thank you