All the champagne yeast I've bought has been dead in the package. Unhook the diffuser, put the airline into some water or something so you can check if it's bubbling. Wait a day for the champagne yeast, if nothing happens then your yeast was dead.
I've done the same thing. Made a batch, added champagne yeast, nothing happens 24 hours later. Add some bakers yeast, within a few hours it's going..
That said, I have one of the commercial CO2 reactors, and I have to crank the crap out of the lid to get it tight enough so that it doesn't leak when dealing with the pressure of a CO2 diffuser.