i experienced how you felt before and the best thing to do is to get your mind off that fear like what was already mentioned. Some of the things Odahviing mentioned is what I do when the fear comes back. Anxiety can give you many uncomfortable feelings that you don't want so if you end up feeling really uncomfortable the best thing is to do something you like for a while that will take your mind off the fears and relax you. Than try falling asleep again using whatever method that works best for you.
^^ Don't worry that fear goes away as soon as you start getting yourself comfortable with sleeping again and think about the times where it felt good to have a nice nap/fall asleep.