I have a translation exercise to do, but I can't figure out exactly how to translate this sentence:

Катя спросила у Лены, что ей делать?

I know it's "Katya asked Lena..." but I'm unsure on the second part. Who is 'ей' referring back to, and what does it mean? I'm sure this is rather elementary, but I can't get my head around it.

Thanks in advance.