If your coil is freezing up in the furnace, then you may be low on freon.
I had this problem. I have a leak in the unit some place, (seeps out an ounce over the Winter and cant find it yet, guy said to wait til it gets larger..sheesh). Every year, or every other year I have to get freon added so that it works all that year...
So, check and see if your coil in your plenum is frozen, or, if its dirty even (electronic air cleaner not working properly, or not cleaned every month, or the filters not replaced enough to get the dirt crudded in the coils...then it needs sprayed out with a special spray you can buy yourself and clean them).
Could also be a broken wire at the thermostat?? How about your breakers for the A/C? Wife called me at work, and told me that the A/C wasnt working, I called the repair man... found out the breaker wasnt on.. 40 bucks down the drain for nothing... sheesh! (Ya cant do a thing, from work, but let the experts at it, because a wife is not usually savy enough to know what or where the breakers are!)...
Ok, my two cents for what its worth, and a wee bit extra for the collection plate