My niece's birthday was on Sunday and I couldn't contact her.
Why couldn't you contact your niece?
She was driving and she couldn't answer the phone.
It's good that you wanted to talk with your niece on her birthday. I'm sure she was busy and just forgot to return your call. She couldn't answer the phone while driving, just call her back and wish her a happy birthday, she'll appreciate that.
That is understandable, she doesn't like to talk and drive.
You are kind to remember and wish her a happy birthday. She'll remember that thoughtfulness and wish you one back one day.