I agree with all the previous posters. Periodically, my 4-year-old will say, "But what if I can't sleep?". I don't know where this comes from, but he usually does it on nights when he's not exceptionally tired at bed time. Then he might pop out of bed a few times and we just put him right back in bed.
I think the situation is probably a little different in your daughter because she's older and you say you've never had this kind of problem with her. I think morah is spot on when she says that you need to investigate what brought this on. Did she see a scary movie by accident? Hear a scary story from one of her classmates at school (that was always a big one with me)? The imagination of a 7-year-old can be phenomenal and can cause no shortage of imaginary terrors. I remember hearing the entire plotlines of movies like Nightmare on Elm Street and Friday the 13th at school from classmates who had been allowed to see the movies (at, like, age 7/8!) and it seriously kept me up at night for weeks--and to this day I haven't seen those movies.
Whenever my son starts the "What if I can't sleep?" game, I remind him that he can take a deep breath, and snuggle down with his blanket and pillow and Tigger, and turn over, and close his eyes, and go to sleep. I also remind him that if he needs us, we're right down the hall. The key is to keep it relaxed and calm.
If she's not able to get to sleep because she's scared, then some reassuring is in serious order and this might take a long time. If I watch horror films (especially ones involving ghosts), I won't be able to sleep well for weeks even though I'm over 30 and I know none of it's real, but my imagination can get the best of me sometimes (ask me about the movie Thirteen Ghosts. That movie still freaks me out). You might have to reassure her every night for a few weeks that you won't let anything hurt her until the memory kind of fades. If this is the problem, though, please address it now. My parents never really did with me, assuming, I guess, that I would just grow out of it, but I was well into my teens before I could even sleep in my own room in the dark (yeah, I'm a wimp about the dark).
Putting her to bed super-tired is also a great idea because she's going to be too tired to worry about not being able to sleep.