Anyone who has a daydream, an empty word document, or both knows what writer's block is. The story is flowing along, the plot rushing through your mind, the right words coming... and them BAM, you are stuck. Your protagonist is out of character, you've accidentally copied Doctor Who, there are six enormous plot holes ripping your story to shreds, and the world is a dark and joyless place.
Do not despair!
There are various types of writer's block, each with different solutions. Several are covered here. But I must say the words all writers dread: Just writing the damn thing is usually the best solution.
Writer's Block Type 1: Plot Problems.
Suddenly, you freeze. You've worked and worked to get here, and you're at the big scene, and then you realize that a lot needs to happen before your character can get out of this. That army that showed up out of nowhere? You realize that "showed up out of nowhere" isn't going to sell. Protagonist is gifted with wonderful powers? Well where did they come from? Here are some tips:
Pinpoint the problem and narrow it down. Often, a broken scene has only one or two sentences making it unbelievable. Find them, and ELIMINATE.
Locate a human being. Get them to listen to you talk it through. Someone who will make suggestions is ideal. Sometimes the suggestions are so laughably awful that you see the light. Sometimes they really help.
Try switching the perspective. This can open the scene up to new information, and hide things you don't want revealed until later.
If all else fails, delete and rewrite. Look at your outline. How important is the scene? Is there somewhere else the needed information can go? Would it work better as action instead of narration? Grit your teeth and rewrite. Don't delete the old copy; put it in a "deleted scenes" graveyard doc.
Writer's Block Type 2: Your Motivation has Perished.
You know what needs to happen. Everything is all planned out. The scene makes sense, you're ready to roll, and... you look at memes instead of writing. Join the club.
Take a breather. Are you sick of your story? Read something unrelated. Work on another hobby. Pet a small soft animal. It helps. The breather can range from several hours to around a month, in drastic circumstances.
Go for a walk and think about your characters. Think about your characters being in the same place you are. How would they react? What would they feel? Go home and try writing again.
Write self-indulgent fluff. Are you a sucker for romance? Write that "holding hands in the rain" that's floating around your brain. Do you like action? "Suddenly... NINJAS!" Do you love description, but always cut it out? Describe that sunset in excruciating detail. It doesn't have to stay in the main draft. Put it somewhere else if you like. But try it.
Writer's Block Type 3: Your brain has forgotten what a "word" is.
Your dialogue sounds wooden. Your sentences are clunky. You are telling, not showing. It's a horrible mess, and you know it, but your brain has never seen a "word" even once in your whole life.
Try writing at a unique time. If you're a late-night writer, go for the morning, or vice-versa. Alternatively, get a good sleep for once and try again tomorrow.
Change the weather in the scene. It helps, no one knows why.
Read. Read stories or writing advice or essays. Find dialogue in books and find what makes it flow. Study a little. Do your research.
Writer's Block Type 4: AAAAAAAAAAAAAAAAAAAHHH!!!!
AAAAAAAAAHH!!!
Solution: word-smash whilst screaming. Guzzle caffeine. Sleep for ten years to work off the stress.
And that's all. Hope it helps.