What happened to Dr. Who's granddaughter?

So I was watching the earliest of the early episodes of Dr. Who with
my friend and I noticed that the Doctor had a granddaughter, who's
existence actually kicked off the pilot episode. This confuses me,
because in the newer stuff the Doctor. . . doesn't have a
granddaughter. I'm assuming that since the Doctor is the "last of the
Timelords" that she died, but I wonder if it was a case of "character
goes upstairs and is never seen or mentioned again".

Anyway, what happened to her?