It is not precisely clear what is the last thing Keller remembered before her illness. After being ill, she remembers the light getting dimmer and dimmer before going out altogether. Most importantly, however, before getting sick, Helen learned and remembered the word for water. After her illness left her blind and deaf, she would still say the word "wah-wah." As she expresses it:
Even after my illness I remembered one of the words I had learned in these early months. It was the word "water," and I continued to make some sound for that word after all other speech was lost.
It's difficult not to imagine that some dim memory of the word "water" helped trigger her years later to make the connection between the water from the pump running on her hands and the word water Miss Sullivan was writing in her palm, unlocking for Keller the key to language and knowledge.
Since Keller lost her capacity to see and hear at nineteen months, the fleeting sensory memories of seeing and hearing occurred very early in her life. One has to think that in a child whose development is not interrupted, those early memories are obscured by the rush of sensations the toddler continues to experience while learning language. Helen Keller had only a few scraps of those sensations to cling to, but they were helpful to her.