In reply to Cappa: It's called an off day. Maybe a result of overtraining (a hard weekend in the hills perhaps?), the onset of a cold, lack of nutrition (over a day or two), or stress (mental attitude).
I have them sometimes. Used to fight through but now realise that I'm better off stopping and saving my efforts for another day. Doesn't happen often but is usually as a result of overtraining and not eating enough.