Well, this is always very tough for parents and family, but very common and normal. Children change their sleeping habits a lot in the first 1,5 or even 2 years. And after that (napping, no napping etc.) Crying is as usual the expression of I am not okay.
A bit more information would be good however for a more specific reply. How was it before? What is the state of the child when waking up? How does she fall asleep? And where?
I will take the assumption that not everything is the same for the child when it falls asleep as when it wakes during the night. E. g. someone was with her when she fell asleep and is gone when she wakes up. Or the child fell asleep in another room and is then put in her bed.
All these things are disturbing when the usual waking up or light sleep phases arrive. (Adults have those as well).
When the childs senses find something that is not as when she or he fell asleep an "alarm" might go off. (Imagine you fall asleep in your bed and during the night you wake up on the kitchen floor, you'd be fully awake within seconds).
It is best when kids learn to fall asleep in their beds, so during the night they can fall asleep on their own as well.
For a method to train this see here:
This book helped me a lot:
The most important according to me is a good night time routine:
Get calmer till the end of the day, do the same things in the same order. The child will naturally understand / know that it's about sleeping time, because it will go to bed after all that. The falling asleep on their own can be tough.
However most important again: consistancy. The situation when the child wakes during the night should be very close or the same as when falling asleep.
All the best and good luck to the patents and further family.
BTW: This can also be due to pain from teething. But usually this is already uncomfortable for kids when they try to fall asleep.