Heater it's dead

I have a chevy cavalier 1997, 120000 miles.
the problem it's that I took it to a repair shop to see why the motor was making too much noise. They ended up replacing the motor. Everything works fine, but this morning the heater won't turn on, don't blow any air. I check all the fuses and they are ok. Took it back to the shop and they said they don't know what's happened. Can you help me?
I'll appreciated it very much.