Recently, Cryptednets.org lost the AC in the server room. I assumed the evaporator coil was filthy, and had frozen. I began to dis-assemble the plenum of the furnace, to remove the A-coil asembly for cleaning, and saw that it was spotless.
(We must do a pretty good job of replacing the filters ;) ) This realisation, however, did not make it any cooler in here.
Anyway, long story short, here’s what happened. The coil *had* frozen, after a 95-degree day, and the high-pressure limiter switch on the compressor had tripped. I went outside to the condenser unit, took off the access panel, and pushed the reset button on the compressor inside the unit. (after turning off the breaker, of course.)** Replaced cover, closed up the furnace, reset the breaker, and viola! Arctic-ness.

All without the $250 service call.

**IMPORTANT NOTE: 240v AC at ~60 amps can kill you rather quickly. Consider yourself warned.)