At workshops I present, I often meet people who can't get past the first fifty pages. They need to make those initial chapters perfect first, they tell me, and then they can go on. That's probably never going to happen, hence a completed MS will never appear.
Still, when I'm stalled, I usually print off what's done and do an edit, which serves a few important purposes. First, it gets me back into the story. I may have stalled because I'm worrying about presenting a character or getting a scene just right. Reading the whole thing from the beginning gets the momentum going again, and the story begins to move forward in my head.
Second, a read-through of "thus far" points out holes in what the reader needs to know in order to travel with me on the journey. The reason I'm stalled may well be that I didn't set things up well enough at the outset, so the event(s) that need to happen don't make sense.
Finally, I get a sense of pacing, mood, and tension. Are things building nicely or plodding along? Have I chosen and settled on a dominant mood, or is the story's tone out of whack? Does the pace rise and fall logically or have I stopped to explain things that, like a river at a curve, drop silt and clog certain points?
I used to tell my students in drama class who needed to memorize a part in a play that starting at the beginning every day is counterproductive. You waste time re-hashing what you did yesterday and tire before you get to what's new. However, when doubts creep into your writing, it's probably your brain telling you that what's already on paper needs work. If you just can't figure out where the story needs to go, reading the whole thing will provide a sense of why you feel that way. So edit: not every day, but certainly "as needed."