Ah, that is rather sad. Thing about Rain spiders is they do not live long at all. +/- 4 years or so. And generally after their 2nd or 3rd time they breed they die. So I don't think it's actually a lack of food as they live for long periods of time without any food. The best I can suggest is to leave the sac for a while, the mommy is supposed to open the sac but i'm not sure as to what satge of devolpment the babies are. You can open the egg sac and might only find eggs, or eggs with legs which need incubation to molt into fully formed spiders. It's a 50/50, up to you though, you can take it outside and cut the sac open, maybe there are little ones in there, maybe it's a dud. Can't really say untill you open and find out. I'm not sure, but depending on the conditions the egg sac was kept in, Humidity, temp etc etc, generally after a month or month and a half there should be small spiders in the sac.
Well let me know what happens if you do decide to cut it open, maybe some other members can give advice aswell, i'm no rain spider expert.