She used to be fine going to sleep by herself. Then around Thanksgiving, nothing changed mind you, she began refusing to sleep unless my husband or I was in the room. We have tried changing her bedtime routine, skipping naps, a night light, and she still screams and cries for us if we try to leave before she is asleep.
Having 4 children myself, I've noticed that nothing has to change for them to decide that something they have been doing is no longer necessary. Although, many times something has changed but as parents we don't think the kids will notice or we have yet to discover the change ourselves (like a disagreement between parents). It is a normal thing for children to test boundaries as well.
With two of my kids, they did not want to go to sleep without either my wife or I in the room. What I ended up doing was taking a migratory approach over a couple weeks.
- I started by not sitting in bed with them while they went to sleep.
- A few days later, I staged myself near the door with it open.
- A few nights after that I stood in the doorway in clear view so they could see me.
- In the final phase, I would wander out of sight for brief periods and then reappear, reinforcing the idea that even if they couldn't see me, I was still there.
Note: Don't expect a perfectly steady progression. Each step will take time and regression can be expected here and there. Also you might find yourself constantly reassuring them that it is ok.
All that said, I'll leave you with this. The period of their lives where you can be with them while they go to sleep is precious and very brief. I'm not advocating sitting there with your 8 year old while they go to sleep (extenuating circumstances excluded). I am saying that if you look back and spent a few years holding your child's hand when they went to bed, you will look back with fondness, not regret.
For whatever reason your child is feeling just a little less secure than she was, and wants that extra re-assurance of your presence. I would suggest simply stay with her until she goes to sleep. Don't try to fight her on this. This stage will pass. It won't take very long.
You say that "nothing changed", but something obviously did, even if it was only in the mind of your child. Maybe she saw or heard something that scared her or caused her imagination to run rampant. Maybe she discovered the fear of death. Maybe a friend lost a parent or relative, and she fears the same will happen with you.
The bottom line is that she feels insecure (afraid, uncertain, worried) being alone in her bed at night for some reason, and I think the best thing you can do is to be with her to allay that insecurity.
It isn't ideal for you (bedtime takes a lot more of your time if you have to sit/lie with her until she's asleep), but it is ideal for her, which is what matters.
Once she feels completely secure with the fact that you are there and taking care of her, she will wean herself from this need. In fact, don't be surprised if, after a while, she suggests that you don't need to stay with her till she falls asleep.
This could take a couple of days or a couple of years, but investing in that trust bond with your child is invaluable, and worth all the additional time spent right now.