What causes the AC blower not to come on?

What kind of car is is it? Without knowing anything else, I would suggest checking the fuses and relays. It should be the same blower; does it work with the heater? If so, it could be an issue with the a/c - heater control in your dashboard. It could also be a bad blower. If you can get to it, you can check to see if there is current going to the blower with a mulitester. If there is, and it's not on you know it needs replaced.