You've probably already tried this, and even so I'm once again struck by my chutzpah at presuming to give tips to a published author when I'm just a fic-writer (with a couple of "I got stuck on the plot" half-finished stories of my own) but -- one thing I know helps quite a few of us ficc-ers is to present what you've written so far to someone you trust, and ask them for input. I've done that for other people, and have asked people to look over my stuff too. They can say what comes to mind, in their mind, as to "what happens next" or for overall plot ideas. Or if you have a general idea, but need blanks to be filled in ("I want to get to Z, but I don't know how to get past XY first") - they can do that. You can have a little brainstorm session with someone, even better if you can do it in a chat (or in person) -- Have you tried something like that?

How about -- reading stories where you've thought -- "I would have handled that differently" or "I wish they had expanded on this idea" -- could you use something like that for inspiration?

Otherwise, sigh, I'm generally of the 'don't force it' variety. And I'm sure you've probably tried all these things already.

So, now that I've babbled pointlessly, all I can say is -- I'm sorry to hear you're frustrated, and I hope your Muse cooperates soon.

Debbie