Your information about the problem isn't clear, Doug. Does the fan come on at all with the air not being cold or is the cracking sound coming from the thermostat itself? I need more info before I can help you solve the problem. Thanks!
Check the blower itself. There is usually a fuse. It may have been blown. If you're not wanting to fiddle around looking for it—it usally looks like a car fuse—then simply call a service technician to come out and take a look at it.