The opposite of hypergraphia is writer's block, and Alice Flaherty of The Midnight Disease has a nice definition of that: the inability to make yourself write plus an active suffering from that inability. The suffering is important. If you can't make yourself write and feel vaguely guilty about that but, on the other hand, it frees up more time to play Worlds of Warcraft -- then you haven't got writer's block.
You haven't got it if you're too unmotivated, too busy, too lazy, or without any ideas. If you do have it, it feels terrible: You sit down at the computer and try to write, but instead undergo depression, anxiety, or even active panic. After enough of this, even the thought of writing produces a sort of mental seizure, which only makes the block worse.
What causes writer's block? Flaherty explores a lot of different theories, including the Freudian, which Freud explained thus: "Analysis shows that when activities like playing the piano, writing, or even walking are subject to neurotic inhibitions it is because the physical organs brought into play -- the fingers or legs -- have become too strongly eroticized...writing, which entails making liquid flow out of a tube onto a piece of white paper assumes the significance of copulation...The ego renounces [this] ...in order to avoid a conflict with the id." Oh, that Freud!
When a student tells me he has writer's block, I want to know how much he's trying to write and how much he's suffering from not doing it. Almost always, one of two things is happening: Either his critical faculty is pounding away overtime, making him so dislike anything he writes (It's not Tolstoy!) that it's painful to continue. Or, more benignly, this particular story has taken a wrong direction. In that case, he needs to go back to the last place in the story where he was enthusiastic about writing it, and rethink from there.
In my own case, I suffer from not writing when the work is either going wrong temporarily (fixable) or is just too ill conceived to succeed (not fixable). It's really important to figure out which. And, if the story or novel is really hopeless, to get started on something else as soon as possible. Get back up on the horse, and ignore the Freudian implications of that.