...Well that just broke the quest completely. I really hope you didn't save after using that key there because if so you've found an unwinnable situation.
You were supposed to take the other route, hit the invisible switch, and then go to the path south of the Zelda tablet.
Oh dear, that is problem. do I have to restart the whole game?
There are many areas where you can do that and also areas where you fall like the example below