Standing on top of something very high and realizing suddenly that there are no hand- or footholds, nor any barrier to stop you falling
An eye at the keyhole of the dark, windowless room in which you are locked
Waking up to find that neither your friends nor your family have any idea who you are.
Being forced to speak in such a silly voice that hardly anyone can understand you, and everyone laughs at you
Which nightmare would frighten you most?