What was the source of the fear and trauma that my client experienced when she woke up in the middle of the night at the age of 12, and now has trouble remembering?

As you saw intuitively, this was an alien abduction that took place. The reason for the memory lapse is because the memory of the event was suppressed. This is to be expected with such events, that every attempt will be made to wipe the memory clean. This, nonetheless, was a trauma for which she needs assistance. There is awareness in the deep subconscious it has taken place and is a cause of great disquiet within her, and is the reason she is bringing it up now many, many years later.