I've posted in the past about the trouble I've had getting lemonade to ferment. It has to do with the acidity.
You can try again with new yeast- the best thing to do is use the same type of yeast (I used wine yeast) and rehydrate it in a little warm water. Add a tiny watered down bit of your lemonade must and a little sugar. Keep doing that over the course of 2 days or so, gradually making it more and more lemonade mixture and less sugar and water. When you are adding 100% lemonade must, and it's still fermenting, go ahead and pitch it.
No need to cover/airlock it. It really needs oxygen during this time. You can cover it with a muslin cloth, or dishtowel to keep out the fruitflies, etc.
If you do a search on this board for "lemonade", you'll find the threads that talk about all this. I think its on the wine making area, but not sure.