In this story, we see evidence of three different methods for entering CAL's cyberspace world. There is CAL herself. There are the 4,023 (including Donna) who were physically teleported. There are the expedition crew members (including River) who had their mental states recorded.
The ones that were teleported in could be teleported out. All the information for restoring their bodies existed in the Library's computer systems.
The ones that stayed behind were different. Their bodies were never recorded -- only their minds. It's not just River. Miss Evangelista, Anita and both Daves were also recorded through the psychic links in their communication devices. None of them have bodies to which they could return.
None of these people can be restored to a physical existence in the same way as the 4,023. For them, there is no interrupted teleportation which can simply be resumed. We do not know that it is impossible to restore them to some sort of physical existence, but we do know that some other method would have to be used, and that someone outside of the Library's computer system would have to do the physical work. Since the Library is now under quarantine, there isn't anyone in the physical world available to help them. Other than the Doctor himself, there might not be anyone who is even aware of their predicament.
For River to leave the Library's computer systems, someone will have to build or grow her a new body, and then download her mind into it. That seems possible, but it doesn't feel like something that the writers will do anytime soon.