Please help..there's something wrong with my rat!?

Hi, my sister rng me earlier today (around 4 hours ago) and told me that there's something wrong with her rat. My rat and her rat stay together in their cage. She's been sleeping all day and she can't stand up. She keeps trying but she just rolls around and wriggles her legs. I pick her up and she just lays there. She doesn't move. She's only 4 weeks old and she's just a baby. Please help? We think one of her legs or arms may be broken or she might have internal injury because it looks like she's fallen off the top level of the cage. My sister found her with her tail wrapped around the ladder. Does anyone have any ideas? She looks like she's in a lot of pain. Please!