I'm so sorry. Losing our babies is so hard for us. The only way to know what happened would be to get a necropsy done. And since you have other gliders, this would be highly advisable. Please take the other gliders to the vet for a check up and fecals to make certain they are ok.
Gliders are so good at hiding illnesses. Most of the time, when they show signs of being ill - they are very, very sick little animals.