'I could not get hold of her' is standard, neutral language.

After reading Su.Ki.'s answer, I would like to change mine. I was thinking of a context of physically holding a person. My answer would be appropriate for that, but really it is not as common of a context as trying to reach someone via a communication line, like a telephone, or email. For that context, Su.Ki.'s answer is better.

I could not get in touch with her. I could not reach her.

It is a little informal. A more formal way is "I could not grasp her", or "I could not grab her."