Awake for hours

I don’t understand. My babe will fall asleep on her own before naps and bedtime but if she wakes in the middle of the night forget about it! She will just cry and cry. And it will take hours to get her back to sleep. Like if she wakes up at 3 I know I’m not getting her back down until at least 4:30-5. So exhausting. Why does it take so so long!