[Von Himmel]

They sure can, but mental trauma with magic sometimes last long. Basically what makes them unable to use magic is because of those memories of failure. In magic, once you had the impression that your magic will goes wrong then the magic will not appear. Maybe it's easy to say that you've recovered.. but as long as the memory of when it fails still exists then it might ends up hampering your magic.

Anyway it's like a trauma. Let's say that someone afraid to get out from his house because something happened in his past and that makes him scared to go out. Of course, it's unlikely that event will repeat itself but this man was too traumatized that he refused to go out even when everything is perfectly safe. In Mahouka's magic, this line of thinking is very bad for magicians.

I think it's implied that Mikihiko also has similar incident long ago that made him lost his ability. Of course most of them are back now (and he's even getting stronger than before too) based on what Erika's saying so I think it's safe to say the inability to use magic is curable.