A 5 year old girl is afraid of the dark, to the point where she screams whenever the lights go out.
How can we help her get over this fear?
I have a few ideas based on personal experience; the main focus is dealing with it as a gradual change, and keeping in mind that all children are different, so something that works for one child may not work in this case. Keep in mind that if she's that afraid at 5, the process may take a while - it needs to be done at her pace, not yours.
Get her a nightlight - she needs to know you're taking her fears seriously, and that you're on her side and want to help. It's a lot easier to help if she's calm and not screaming.
Talk to her about her fears - a five year old can usually carry on a focused conversation well enough to tell you more - is it monsters under the bed? Scary noises? Pile of laundry that looks scary in the dark? Or just that she can't see? Each of these might clue you in to an idea that could help (checking under her bed regularly with lights on, showing her regularly what's making that strange noise, etc). You might find that she's more comfortable in other places/areas of the house as well.
Let her experience it gradually getting dark - as it gets closer to winter this is easier without keeping her up late - watch a few sunsets, or sit around playing in the evening with the curtains open as it gets dark outside - try to delay putting on lights until she asks, and once in a while try to keep the lights off longer as long as she's still comfortable. Try to play under blankets or an indoor tent where it is a little darker.
Read her stories about being afraid and overcoming fears - head to your local bookstore and ask for recommendations - perhaps based on fictional characters she likes (Dora? Elmo? Winnie the Pooh?). Hearing stories about other fears and overcoming them (including your own!) will help her see she's not alone with her fears.
These may spark other ideas to try - the key is to stay supportive and go at her pace.