E.K.
My son did the same thing, he would sit up in bed and not know how to get back down, we also had the same problem when he learned to stand. I read a lot about it because he would look so scared when I'd go in to help him get back to sleep. It's just that she now has a new skill and wants to try it all the time, even in bed or in her sleep. She just hasn't quite mastered the getting back down part. I would go in to my son's room and let him know that I was there and show him how to get back down, by stretching his legs out and laying him back down, when it comes to standing you will need to show her how to bend her knees to get back down. It took a few times but he soon had it mastered. We had this problem for maybe 2 weeks, it lasted longer with the sitting up stage since he was more mobile by the time he could stand, he didn't mind plopping back down in bed so much. I hope this helps and before you know she'll be on to something new!