What do I do if a mother dies before weaning her litter?

This may sound cruel, but sell babies for snake food if you are not willing to stay up long hours and hand feeding babies or there is no foster mother available. It is kinder then the pain and torment of the babies starving to death if you can't feed them often enough. Or, if you have the time and patience, you can try to hand feed them. We use 1/2 cup 60:40 evaporated-milk:water mixture with 1 tsp. light corn syrup (Karo). Draw 1ml. into needle less syringe (MJC), heat.
Test on forearm ... Feed one drop to each baby in hand. Repeat for entire litter. By second day they should be holding syringe like baby bottle. Stroke lower abdomen with wet finger to stimulate evacuation. When they can crawl, put Gerber Baby's First Rice mixture into heavy glass furniture-caster cup. Also graham crackers or monkey chow soaked in milk mixture. Next feed non-fish wet cat food. They will love you forever.
Goat milk can be used instead of the evaporated-milk:water mixture. Honey can be used instead of corn syrup in a pinch.
Babies losing all their fur when hand raised is normal. It will grow back unless they were supposed to be hairless. :)
Your best bet though is to foster the babies onto another mother rat when available as hand raising is not always successful or practical. Mother rats are pretty good about accepting strange babies. If mixed well with their own, they can't even tell them apart if they all smell the same. If the mother was lost due to respiratory disease, don't risk infecting a foster mother or other rats. The best thing is to euthinize the babies to protect other rats from exposure and to prevent their suffering, as they will likely die anyway and the intensive handling of the sick babies to hand feed them will help spread the virus.
Webmaster note: I took in 3 orphans once who, unknown to me were carrying corona and KRV viruses. Not only did those babies die, but I lost 15 other rats by the time the virus swept through and half those who survived were left with damaged lungs. I know from personal experience that sometimes one has to be a little more hard hearted towards orphaned baby rats.
If you have no foster mother available, call around to other ratteries and see if they do. I found its best as a breeder to breed females in pairs so I always have an extra 'mother' if needed.
Note: It is very important when feeding to be careful to not feed to quickly or the formula/milk may end up accidentally inhaled. The most common reason for death of hand-raised baby rodents is pneumonia due to formula getting in the lungs.