I'm so sorry for your loss.
It's really difficult to guess what happened, it could be a disease (gliders are known to hide their symptoms) or an accident (a bad fall for example).
It is not uncommon for gliders to eat the dead members of their colony in order to try to hide them to not attract predators.
I'd suggest to have the surviving one checked by a vet if possible to make sure there is no disease before introducing new gliders.
In the meantime, try to keep your glider stimulated, bring more toys and try to interract with the lonely one as much as possible.