Out of Sight

When Ivy wakes up with no recollection of where she is, she finds herself literally living her worst fears. From being afraid of the dark, all the way to the part of her past she had blocked from her mind; there are no limits.
But all she seems to recall is a birthday party, although she wasn't sure who's. Just that it was important. And there was a gun shot before everything went dark.
Who's birthday party? What was the gunshot for? Where did she go afterwards? Why is she in captivity? And how can she get out?