car can't start?

[dvdevo]

i recently got a tuneup after i bought the car, and the guy changed the cat converter. but the first time i tried they had it working, then i tried starting it up and it didnt work. So i left it there. The next day in the morn they said it worked (without changing anything) now after 2 days of driving the car, it doesnt work anymore. it wasnt like this when we bought it. Can the cat converter prevent the car from starting up? this is a 89 hatch with 114k miles. thanks.