Hi C.,
My oldest daughter was daytime potty trained by 2 years old, but she had trouble at night up until 4.5 years old.
She was very upset about it by about age 3, so we asked our pediatrician about it and he said that there is some chemical in the brain that signals the body to stop making urine at night, and really little kids sometimes don't produce this chemical, or not enough of it. Also sometimes their bladders are small in proportion to their bodies as they grow. In other words, it is not always within the child's control to be night time potty trained.
With our younger daughter, she became daytime and night time potty trained all at once! We didn't do anything special to make it happen, she just apparently has a big enough bladder and enough of whatever chemical in her brain to be successful with night potty training.
So, you could do one of two things. You can wake your child up in the middle of the night and sit her on the potty, or you can put her in pull-ups at night. I picked the pull-ups option because I figured that sleep does more for me and my child than staying dry at night! They do eventually become night time potty trained, don't worry.