Big Scenes and Uncertain Words

Writers often run into a wall right in the middle of a 'big scene', and even more so within a key scene that could change the direction of the story.
I've come to realize that when this happens, it means I'm making the character do something they really don't want to do. They don't want to kill the bad guy, or they don't want to fight with their best friend, at least not over whatever you had planned, no matter how good the follow up scene to the action might be.
Back up and look for the signs where you started slowing down and feeling blocked. You can usually find the point the story went wrong where emotions started running high.
Maybe your characters really are angry, and that's fine, good for tension. But if they're throwing punches on a weak pretense just so the author can have them make up, it's not going to work.
Same goes for romantic situations. If you're concocting the moment of a kiss or falling into bed together, your readers are going to roll their eyes if it feels forced.

Some words and phrases that can clue you into problems in a story:
suddenly
unexpectedly
unsure what to do next, ...
without thinking
to his/her surprise
for no (discernable) reason*

Of course any of those words can have a perfectly good use within a story. But if you find them in a section you're having problems with, it's time to re-evaluate the scene.
Often they're used when a writer is uncertain what happens next, and, unable to make one scene flow to the next, they force it. And just to make the forcing sound acceptable, they toss in one of those words. A person who suddenly jumps from behind the door and is met with unexpected opposition, is a far cry from a couple in the midst of an argument who 'suddenly' find their lips mashed together, and to their surprise and without thinking fall lustily into bed.
People are always thinking. It's what brains are for.

So, you're characters have deserted you, went on vacation and left you to fend for yourself. Or maybe they ran away screaming after seeing what you had in store for them?
Either way, your story is unfinished and you can't think of what to write next.

Go back to where you were excited about the scene, possibly a paragraph or so before you ran into trouble. Play the 'What-if' game. Go through every possible idea, even the ridiculous, and see if any jumpstart the story back to life.
And of course, you can always ask another friend, writer, or beta reader. They can offer valuable input on what feels wrong and right, and what directions might work to get you and the story back on track.