Did you get the cutscene conversation with Zagan as you approached? He should turn hostile after you automatically transform into a dragon.
If you fly up to the exit tunnel to transform into a human, and then jump back into the cavern to transform into a dragon, does that help? It probably wouldn't, but worth a shot.
Try saving, then teleport to the battle tower (if the dragon stone isn't disabled there) and return, to see if that helps. If you leave by the exit tunnel there is no way to get back there (unless you have the Developer's Cut version of the game, and can use the Developer Mode cheat window's teleporting functions).
