When did it stop working?
The dragon stone is disabled during certain fights or in certain regions, but should be enabled again when they are finished, unless there was a glitch or you exited abnormally. Ie in D2:ED it was possible to enter the bunker at the start of the fjords, start the fight and then fall through the force field covering the hatch, so the dragon stone was disabled and you could not re-enter to finish the fight.
Starting or completing a main plot quest should get the dragon stone working again. Talking to the statues in the waterfall cave at the beginning of the fjords to get the seeds quest has helped a couple people.
Dragon Stone stopped workingIf you don't have any main quests that you can get/complete (not sure if anyone has tried side quests to see if they would also work), email support@larian.com with a description of the problem. With your save they should be able to reset the dragon stone (and/or figure out what cause the problem); see Lar's post
here on how to send them an Xbox save.