sigh.... another pet shop disaster!
I'm not experienced with baby rats, but I think you just need to keep everyone warm and comfy. The mum will need some extra nourishing food, I'm sure people will post and help you with this. I think mashed potato with cream or ideal milk is supposed to be good.
You will probably be able to see milk in the babies tummies, that will let you know if they are feeding ok.
Does everyone look ok? How many babies have you got, can you see?
Depending on where she had them you might want to move them to a safer place - down to the floor of the cage if they are up high
I don't really feel qualified to tell you much more, but I'm sure you'll get a lot of posts from people who have had litters - good luck!