Hi Bella, sorry you haven't had a reply yet.
The most likely explanation is that there's something wrong with the water. In the short term, do a large water change, making sure the new water is warmed and dechlorinated, and she if she improves.
In the longer term we need to find out what might have caused your fish to die, and what might be affecting the other one, so I'm going to have to ask you a lot of questions; apologies in advance for that!
How big is your tank and how long has it been set up? Is the tank filtered and heated? How often do you change the water and/or clean the filter, and exactly how do you do those things? Are there any other fish in the tank, apart from the female guppy? Does the tank have live plants? Do you have test kits for the water?
If you could answer any of those questions, we'll be able to help you a lot better.
