I've definitely had days like that where I just don't feel like doing much of anything! Usually it results in me - similarly to what you described - getting decked out in my workout gear, hopping on the treadmill/popping in the workout DVD/getting set up with my weights for strength training only to give up a quarter through the workout.
It's frustrating. Usually the next day I'm much more energetic + willing to kick it back into gear.
SanityNow said it best: sometimes you just need to switch things up