I have never had that happen to me - and it could be very frightening. But it also could be your subconscious mind letting you experience yourself as pure energy. Sounds hokey, I know, but, for me, I think I would like to see what it would be like - to be just there - not anyone or anything - just there. Try to relax into it, take a look around (inside the space) and let yourself just be - that might allow the panic to subside. Trying to fight panic is useless - flowing with it, looking at it, realizing that you are not in any danger can help (sometimes). Knowing the WHY can also help but if you are asleep first it would be hard to know what the trigger was. Probably a dream or a memory from the past coming back up to be released and free you from something that happened a long time ago.