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Sounds like you have burnt out or are coming near, rest, step a way for a time a week a month don't do years if you can help it but you need to refresh yourself. Watch movies, read other books for pleasure, look into history you don't know well at all. Breaking can allow you some space between the work.
You could take up another smaller project then check back in with the core work to see if you've gotten something new.
Think of when you first made the work if you imagined the ending what was it that made you go "Oh Yes I want to write that!"
You could thought dump. Open a word document write whatever no matter how insane or let your characters speak how they'd end it for you. Nothing is wrong or off or out of move in this format.
You could free write with no context, free of project constraints, a plot what plot if you will. Toss your characters into a situation. What situation? That's up to you or you can random plot generate it and make them undertake what is spat out regardless of what time period or non earth world you have them in. To just get a revive mentally for them.
What was your ideal ending? Are you stopping because your written material no longer fits that ideal? Try re-reading your material what is the logical path/conclusion/adding up of the core beats should lead to now? Maybe it's you can't see the path forward with all those trees. Refined it at least on core beats if not looking at the whole works.
You can talk to people about what you've written and ask them to make up the ending this will involve hours though or you could both co-hash out some scenarios based on core beats only. You have to pick someone open to listen to a lot of made up things if you are writing fiction but if they're open to that you seeing their minds working and eyes lighting up as they play with the material with you is awesome you might get lost just following their flow rather then tying your material down by the origin standards you made.
What do you do when you are out of breath after a run?
The purpose of this question is to get you to adapt strategies or actions you perform after the experience of exhaustion in exercise to burnout from writing. The thing is many writers come to this point without a strategy, then the result of it is a period of rest longer than desired. It’s important to have a set of steps when you run into a road block in order to work through an inevitable moment of inactivity with writing.