I'm with chimchar here, I have a very real fear of the dark sometimes, even at my age, (29) to the point I have woken DH and made him check under the bed, in the wardrobe etc.
FWIW my parents shouted and screamed and punished me, but it didn't matter, at 2am in the pitch black you could have grounded me until I was 50 I was genuinely in heart racing fear when I was younger (and occasionally still do) to the point I would be in tears.
I didn't learn to sleep with the light off until I was 16 and even now the slightest thing can still make it stay on.
Vet everything she watches, all my friends laugh now, I will not ever watch a scary film at all, even some 12A's trigger me, I will not watch them, it's not fun and I don't enjoy them.
Get a bedside light/lamp that you touch the base of, bigger area to aim for when scared, get the ones that have different levels of light, mine has three levels, so you touch once for very dim light, twice more for very bright and once more for off. Let her leave it on, on the lowest setting. Whole cheap range here: www.argos.co.uk/webapp/wcs/stores/servlet/Search?storeId=10001&catalogId=1500002951&langId=-1&searchTerms=TOUCH+BEDSIDE+LAMP
If she has an open bed frame (as in you can put things under it) consider getting a divan, nowhere for monsters to hide underneath (made a big difference for me)
Let her listen to a audio CD, nothing remotely scary though, at all, or read a calming book before bed.
Also I used to tie my dressing gown cord around my wardrobe handles to make sure nothing could ever sneak out just when I went to sleep (I'm sounding like a nutter now
).
Do you know though I have an amazing imigination, and I loved English because of it and reading is such a pleasure, I just recognise that some things freak me out because my imigination is so good, I doubt I will ever change now though and just do things to make life easier.
Please don't let it affect your relationship and all the added stress will make your daughter far far worse, as she will know it's something she should control, but you really really really can't (much to DH annoyance when I wake him from very deep sleep to check something).
Sorry it's long but hope it helps