Claire starts to remember bits and pieces during her memory lapse.  Libby tries to help her by hypnotizing her.  While out on her trek with Kate and Danielle it finally comes  to her how she lost her memory.  Danielle had knocked her out in order to save her from the Others.

[Gr.,=forgetfulness], condition characterized by loss of memory for long or short intervals of time. It may be caused by injury, shock, senility, severe illness, or mental disease. Some cases of amnesia involve the unconscious suppression of a painful experience and everything remindful of it including the individual's identity (see defense mechanism). Retrograde amnesia is loss of memory of events just preceding temporary loss of consciousness, as from head injury; it is evidence that memory proceeds in two stages, short term and long term. To cure amnesia, attempts are made to establish associations with the past by suggestion, and hypnotism is sometimes employed.