Speaking from my experience, I usually come up with ideas first, then make an outline for the story, and then just kinda let my mind turn the outline into the story. At I wasn't able to fill the blanks between the point in the outline with much, and the points that I'd expand weren't too great either, but as I got more and more practice, these segments started to get better too.

Of course, sometimes you're in no shape to write - that can happen, and that's fine. You should beat yourself up about it - that'll only make you feel worse and make it more likely for you to be able to write the next day.