You may have to call in a technician to inspect the units, it is hard to tell as there can be various reasons. How old are your units? Mine at anout 13 years old and I keep thinking of the day we will face a very similar situation.
Sounds like the compressor is out. You still get the air from the vents because the blower fan is in the unit that is inside the house. You need an A/C company to check it out. I suggest CoolRay. Their repairs have a lifetime guarantee.
Here are a few HVAC contractors you can contact from hometalk http://www.hometalk.com/feeds#type=tabs&...