Okay, so you've lost your muse. You don't want to be in your story anymore. You're stuck. And it's not writer's block. You're really trying to trudge through it, but it just seems futile. What do you do? How do you get unglued?
Well, the first thing to do is to determine why you're stuck. And you must be brutally honest. If you can't tell the truth about yourself, there's no way you can find a constructive solution for your problems. And once you take a good clear look, and if you find that your setback is procrastination, then it's a pretty easy fix. Get off your bum and just do it. Anton Chekhov would demand you "write, write, write till your fingers break!" Of course, that would be a little anti-productive – it's hard to continue with damaged goods – but you get the point. The secret to writing success is to create a writing schedule and stick to it – 1 or 2 pages everyday, whatever you can manage. But be realistic and pragmatic, and stick to your schedule like Gorilla Glue - writing is a lifestyle, not just something you do when you feel inspired.
However, if you're stuck because of structure, plot, or story problems, it makes sense to reexamine the main sequences and plot points of your screenplay. Remember, generally, 8 sequences and 5 plot points is a good standard to follow, and if you didn't outline anything at all, get the hell out of the script and start outlining. You don't build a house without a blueprint – not a good one anyway.
But if it's your main character you're struggling with, then you might want to take a deeper look into your protagonist. Use a character questionnaire to see how much you really know about the character. If you're forcing that character to take action simply to fill plot requirements – especially things the character wouldn't probably do – then you clearly don't know your protagonist well enough. Action is character, but the action must be believable.
But then, of course, there is the dreaded SECOND ACT DRAG - if you're stuck because you lost your way somewhere in the middle of the script, a great way to dig yourself out of that abyss is to take advantage of your subplot, and if you have no subplot, go back into Act One to set it up.
Finally, if the reason you're stuck is because you're trying to make your first draft perfect and amazing, you're wasting a hell of a lot of time. First drafts are rarely any good. That's why writing is rewriting. It might take ten or twenty drafts to get it into shooting script shape. So just get it done, and get it done fast. You have plenty of time to fix it later.