Get into a different sport for a while. Take up jogging, swimming, or nordic skiing. Any of those will keep your cardio system in shape, and nordic skiing is actually wonderful cross training for riding. Don't even look at a bike for a while (a couple weeks up to a couple months, depending on the individual) and just concentrate on your new sport. After that bikeless period, hop back on and go for a fun ride (not a hammerfest or grueling ride, unless you think of those as fun) and you'll feel tons better about biking. Hitting a rut happens to everyone eventually (I'm in one right now) but doing what I suggested can really help pull you out of it.