The Legend of Zelda: Skyward Sword apparently suffers a game-breaking bug if events are done out of sequence, keeping you from progressing further. A reported response to a fan from Nintendo outlines the problem, but unfortunately doesn't offer any solution to players who might be stuck.
"If the game has been saved after completing the sequence of events, it will no longer be possible to proceed through the game on that save file," said a Nintendo representative.
As detailed on Zelda Informer, the problem occurs if you talk to Golo the Goron in the mines after receiving the Thunder Dragon's song, but before heading to the forest and volcano regions. The events in those regions become inaccessible at that point, making it impossible to go on.
"The only options at that point are to either start the game over or to use an earlier save file to continue," the representative notes. "I wish I had a better answer, but I'm afraid there currently is no other fix available. I can assure, however, that we are documenting all feedback we receive on this issue."
So an ounce of prevention is worth a pound of cure, but it would certainly be nice if the glitch didn't exist at all. Nintendo doesn't tend to patch its games often, so your best bet is to avoid talking to Golo. Or just all Gorons. Or, to be on the safe side, anyone at all.