Depending on which release you have, there may be a patch for the game (check
here), but there have not been any recent patches, and AFAIK not for this specific issue.
It is likely the dragon stone will start working again on its own, the next time you start or complete a main plot quest (one person had it start working again after they got the seeds quest in the cave under the waterfall), or after the next time you encounter a boss fight where it gets disabled and enabled.
Do you have any saves inside the fortress, before you killed Stone (or after that but before leaving)? Entering the arena likely disabled the dragon stone, but it should be enabled again as soon as the fight is over.
Did you defeat Xanlosch and Isabelle? Did you pick up all the loot there and in the arena?
Did you get the invoker jewel from the chest in the arena? If so, and you did not try using the 4 invoker jewels earlier in the headquarters, maybe return to the headquarters, grab the 4 jewels and use them to disabled the machinery there spawning walking armours. Doing it that way will produce a chest with some random loot.
This shouldn't have anything to do with the dragon stone being disabled, but maybe doing something else in the headquarters (if you didn't previously use any invoker jewels) will trigger a reset.
Try teleprting to the battle tower (master's chambers) and see if you can use the teleport feature there to go do a different platform. If not, take the door to the skill trainer platform (as you did before getting the dragon stone) and see if walking back to the entrance brings up the teleport menu. Don't save there if you have a problem, though, just reload your last save.
Did you try switching regions a couple times (to Broken Valley or Aleroth and then back to the fjords)? That could conceivably get the stone working (probably not, but worth a shot if there are no main plot quests you can quickly get or complete).
If you can not get it working, try emailing support@larian.com, with a description of the problem and maybe a link to this topic. A couple people ended up with disabled dragon stones after falling through the hatch in the bunker at the start of the fjords, and not being able to climb back up, but Larian fixed the saves to put the characters back in the bunker to finish the fight.
In Vista / Win 7 the saves are in the folder below. You may need to set Windows Explorer to show hidden files and folders (see
here).
C:\Users\ %username% \AppData\Local\Divinity 2\Savegames
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