If you've written anything longer than a grocery list then there have been times when the words won't come. For those of us with a day job, not trying to pull the day job card, but I really think it's true. Any time you have to stop then try to start again it's harder for the words to come or maybe you stopped thinking you were already to one point but that is so not where you are-cuz your mind moved the story along without you-that has happened to me more than a few times.
My very first story took a year to write because it sat in a drawer for two months because the words wouldn't come-serious.
Anyway for whatever reason you're sitting there and nothing is happening and the longer nothing is happening the more freaked you get. Deep breath and try a few things first.
1. The one that seems to work best for me is to start reading what you've written so far, it puts you back in the world, now it might seem time consuming if you're at 36k or 55k words in but hey do you want to write or not? You can also edit as you go along, a dropped word or missing, it isn't really a waste of time.
2. So if you're only five thousand words in the above might not help. Okay, was there another scene further ahead you have planned out, write that. Usually by then you're deep enough in your story you can go back to where you left off to start again.
3. Neither of the above working for you? Maybe is the time to write your blog post for next week. Really. Often it's just the need to write something that will get the words flowing.
4. Pick anything from the current story you're writing, a name, a supporting character and write five thousand words about them, from anything to how they're day went to why they are the asshole they are.
5. Desperation, write anything and I mean anything just start typing. Is he in his office? Have him pick up the phone and make him have a conversation. Is she crying over him? Make her stop and go get a carton of ice cream to eat instead. My personal horror story-cuz I am really not used to sex scenes- they are about to have sex and you stopped. Just power through with an off the wall, porno inspired sex scene that might not even make sense. Just start writing and you can fix it when you're done. Maybe the blah scene will inspire a better scene or another story.
6. Nothing.....really? You tried all of the above and nothing is happening? Then maybe you just need to set it aside and move onto something new. Which hugely sucks, especially if you are on a schedule but if after all that no words are coming then maybe right now isn't the time for them to come.