Summer heat - AC - what could go wrong?

The Phactors are not big fans of AC; open windows and fresh air are preferable.  But sometimes it just get oppressive. Yesterday brought the first real heat of summer so before the house heated up too much, the AC was turned on and for a few hours functioned fine.  And then it wasn't functioning at all.  So first you check to make sure it's plugged in; no blown breakers apparent.  Make sure the batteries in the thermostat are fresh; check.  Then you call a technician and wait.  A window fan and the ceiling fans functioned to adequately provide comfort over night. Ah, tech just called so now we'll find our what went wrong. That was damned quick as the call for help only went in 2 hrs ago.