Agreed that the yeast is almost certainly still viable... you mentioned you 'waited too long' so now it's still and sweet... the still I understand as there is no reason it would not be still... but not sure why it's sweet.
I'm thinking you maybe usually bottle while there is still some sugar in there and fermentation is happening, and this time you missed the window. To bottle carb after fermenting dry, the easiest way is just to add some more sugar for the yeast to consume (around 1 ounce/30 grams of sugar per gallon). A tougher way is to add a more 'aggressive' yeast, as others have mentioned.
-Andrew
diyhardcider.com