Pitching yeast directly into wort is not a good idea - it reduces the viability by at least half for normal strength wort, and presumably even more for higher gravity or higher alcohol worts.
You have a lot of yeast in this brew, presumably dead or dying, so I would consider racking soon to avoid picking up a yeast bite in the beer. A schedule like this should help
- make a big starter - pitch some fresh yeast, ideally the same strain from the recipe into fresh wort. 2 liters would be the minimum. If the yeast is dry, be sure to rehydrate according to the packet instructions - this is key to obtaining the highest viability. If you have a stirplate, use that also.
- When the yeast are actively fermenting, typicaly after 24-36 hours, rack your partially fermented wort to a new vessel, to get it off all the existing, mostly dead yeast, and pitch the actively fermenting starter.
The actively fermenting yeast should have a high alcohol tolerance, ideally the abbey yeast from the recipe, to preserve the intended characteristics of the brew. I don't recommend champagne yeast, unless you are sure the remaining wort contains little maltose.
Something to consider is to incrementally add wort from your brew to the starter while it's fermenting to get the yeast gradually acclimatized to the alcohol.
So, in short, rack the beer off the old yeast, and pitch actively fermenting yeast. That should get it started.
