A cause and effect essay will focus on the effects of a certain action. Since your cause is the lack of memories, there are several effects you could focus on. I suggest focusing your essay around three main effects.
First, the lack of memories means the Community has no history. They exist in an endless present, so there is no pain of the past. This also means that they don’t ask questions, because they don’t know things can be different.
Second, the community requires a Receiver of Memory. This is a difficult, hazardous job. If they lose this person, it causes everyone in the Community great pain.
Third, Jonas changes his opinion about the Community when he starts receiving memories. This new perspective, in addition to Jonas’s incredible amount of empathy, leads him to escape in order to return the memories, changing the community forever.