First I just want to say how terribly sorry I am for your lost. I have 9 babies and could imagine losing any of them.
By your post I'm guessing you don't know what happened. With gliders it could have been a variety of things. Gliders in captivity usually live to be 15 to 20 years if cared for properly. I hope you don't take offense to that, I'm not saying you didn't care for them properly, but maybe you do not know all the facts when it comes to these little guys. There is so much misguided information out there when you research on how to care them, and most of it wrong. And then there are times gliders just pass even when cared for properly. Gliders are very good hiding when they are sick, so you have to know what to watch for.
So I'd like to help you figure out what could have possibly happened with your baby. First thing I'd like to ask, how long did you have them?
One of the main reasons gliders pass unexpectedly is dehydration. Gliders can become dehydrated very very fast, and once they are, it only takes hours for them die if not rehydrate right away. Check their water sources. What kind of water source do to use? And you should always have 2 water sources available incase something happens to the one.
What do you keep in their cage? What kind of toys and bedding do you use?
Do you see visible injuries? If so what, what kind? And what kind of wheel do you have?
What diet do you feed them?
And, what kind of Cage do you keep them in?
Posting a pictures of their cage would a huge help as well.
If you don't mind answering those questions, it will help narrow down what may have cause this, and help keep your other glider Alive.
Also, you want to spend a lot of time with your other glider. It will probably become very depressed now that her cage mate is gone. And if she does become depressed she could start to over groom, starve herself, or even self mutilated. Also your going to want to take everything out of the cage and clean everything just incase there is some kind of bacteria that could have possibly killed the other one and also to get rid of the other gliders scent in the cage. Keeping the deceased glider scent in the cage will only at to her depression and make her miss her cage mate more. You will definitely need to get her a friend as soon as possible, but need to do introductions the right way. Gliders are colony animals, so they don't do well by themselves. You might notice your gliders attitude towards you might even change while she is by herself. Spending as much time as possible with her is vital to her health right now. Even if she is just sleeping in a bonding pouch and carrying her around with you. This will help with depression, but eventually she will definitely need another friend asap.
I'm not here to help you. I'm not her to critize, judge, or bash you. My only intention is to help. I hope to hear back from you. If you prefer to send me a private message, please feel free to.