"I had not thought about it."
Translation:Non ci avevo pensato.
I'm not native Italian and my understanding is weak, so someone with more experience can chime in and correct me:
"Ne" = "of them/it" but not "to it."
"Ci" = "to it."
In Italian, you say the English equivalent of "I think to x" when you say "Penso a x," which is grammatical. "Penso di x" on the other hand is not.
So when you say "Ci avevo pensato" you are saying "I had thought to it."
Not quite so simple as that. Consider this (Pensa a questo!):
"Ci penso io" can mean "I will see to it (take care of it/give it thought)."
"Cosa ne pensi?" means "What do you think of/about it?"
"Penso di si/no" (and it is grammatical) means "I think so/not." and "Penso di partire" means "I am thinking of/about leaving."
"Pensare" is a verb with several usages, depending on the preposition, the tense of the verb, the complement, and the context. (But you are right about the difference between "ne" and "ci")
The following link to Reverso Context might provide some help on where you might use pensare a ciò