It could be Voldemort. Voldemort might use Harry's love as a weapon against him. He could either kill her or turn her against Harry. Another possibility is Draco Malfoy. Maybe Draco tricks Harry's love into thinking he's a better choice. Or it could be a new, unknown dark wizard who targets Harry through his love.
Well, one possible plot could be that Harry's love is taken away due to some dark magic. Maybe an enemy casts a spell that makes his love forget about him. Another could be that Harry has to sacrifice his love for the greater good, like to save the wizarding world. And also, it might be that there's a misunderstanding between them that causes the loss of love.
Often in such fanfictions, the plot may involve a betrayal that pushes Harry over the edge. It could be a friend or a mentor who turns on him. This sudden shift in loyalty would shatter his world view. He has always relied on the support of those around him, and when that is taken away, it becomes difficult for him to stay sane. His actions would then become erratic as he struggles to come to terms with this new and cruel reality.
He could regain it through the support of his friends. If Hermione and Ron show him unconditional love and support, and remind him of all the good he has done and all the people he has saved. Their friendship has always been a strong bond in his life, and it can be the key to bringing him back from the brink. Also, if new friendships are formed, like with some previously overlooked characters who offer him new perspectives on life.
In 'Harry Loses His Emotions Fanfiction', it could be that Harry experiences a really traumatic event. Maybe he witnesses the death of a very close friend in a battle. This shock is so great that it makes him shut down his emotions as a self - defense mechanism. He then becomes cold and distant, and the story could be about how his friends struggle to reach the old Harry again and help him heal.
Well, he might feel extremely lost and out of place. His identity has been so tied to his magic throughout the series. He could face a lot of ridicule from those who once feared or respected him for his magical abilities. Maybe he'd have to rely on his non - magical skills like his courage and loyalty more than ever.
Maybe in the fanfiction, Harry's twin could be lost in a magical accident. For example, during a wizarding battle, the chaos causes them to be separated, and Harry spends the rest of the story trying to find his twin.
One possible reason could be that he's constantly being underestimated or not given the credit he deserves. In some fanfictions, his friends might take his contributions for granted, which could lead to his temper flaring up.
Well, maybe he stumbles around blindly for a while, bumping into things and getting into all sorts of comical situations. He could also rely on his friends more to help him get around until he finds his glasses.