Oh dear.
Bam and AxGizmo have given some excellent resources for you. I hope that helps.
I would let the shelter know. Im not certain if that would achieve much but still, let them know regardless. In an ideal world, shelters would only let desexed animals be adopted out. Because this shelter one didn’t and now a burden has fallen on you which you certainly didn’t sign up for.
Do read throught the links provided above and ask any questions you have.
ETA: Its quite normal for the mother not to hang around the babies. They may only visit briefly to let them feed. I read that they feed them soon after they are born and cleaned up. So she may not provide another feed until a day has passed. Do check that the babies are pink, warm and healthy looking.
ETA2: I really dont know how accurate that is regarding the feeding. It may vary rabbit to rabbit. I seem to remember Maple a few days after giving birth got a little sensitive (hormonal) again and I thought that was when her milk came in… She only had one baby to feed though so it could have been different for her then the norm.