Voldemort might initially be furious. He's not used to being challenged in such a blatant way. He could try to destroy the Howler immediately in a fit of rage.
The Howler might contain Harry's anger and defiance. He could be yelling about all the harm Voldemort has caused, like the deaths of his parents and many others. It could also include threats of how Harry will stop him once and for all.
It could lead to all sorts of chaos! Maybe Voldemort is furious and retaliates immediately.
Perhaps it leads to a confrontation between Harry and the fans, or it could inspire a change in the way the fans view Harry or Voldemort. Maybe it even sets off a chain of events that affects the entire magical world in the fanfiction.
Maybe Harry was really angry and wanted to shock Voldemort. A howler is a bold way to show that anger!
Harry might immediately gather his friends and share the contents of the Howler with them. He values their input and they've always been there to support him. Together, they would analyze the message and come up with a plan. Maybe he'd be a bit angry too, feeling that Voldemort is always trying to disrupt his life and the safety of those around him.
It could be Dumbledore. He was always one to oppose Voldemort and a howler from him would be a very interesting plot point. He might use it to taunt Voldemort or to try and make him second - guess his actions.
Harry might be initially shocked and embarrassed. A Howler is a very public form of scolding, and in front of all his friends and classmates, it would be a humiliating experience for him.
Well, if Harry got a howler from Voldemort, it might be filled with dark magic and menacing words to unnerve Harry. It could also be a ploy to throw him off his guard and disrupt his focus on fighting evil.
Well, it could be that Harry sends a Howler to the Ministry of Magic. He might be frustrated with their inaction or wrong decisions regarding the safety of Hogwarts. The Howler could expose some of their secrets or simply be a very public outburst of Harry's dissatisfaction, leading to a big political mess within the wizarding world.
Perhaps it's a taunt about Harry's power. Voldemort could be saying that all his efforts to defeat him are in vain because Voldemort is far more powerful. He might mock Harry's attempts at learning new spells and defenses, saying they won't be enough to stop him in the long run.