I too would agree that it is a matter of insecurity. Has anything changed within the family recently? Someone go away for a period of time...traumatic experience? Anything that would make her worried about being alone? Sometimes knowing the cause can help fix the problem.
I would recommend talking to her (assuming she can talk back) and ask her what she's afraid of. Maybe it's the dark...maybe it's a scary stuffed animal in her room. I know my son started waking up at night screaming bloody murder and (since he can't yet talk) I finally figured out it was because his big teddy bear was positioned "just so" in front of his night light casting a very daunting shadow :) I moved the bear and haven't had a problem since. Once we can determine the cause, it narrows down the "trial and error" period. Best of luck to you!
