OK, so I went to go turn on the central air for the first time this summer and I saw a spark inside the thermostat and what do you know... the A/C won't turn on, and neither will the Heat. I replaced the old (and I mean OLD) thermostat with a new programmable one, and still nothing works. I KNOW I wired the new thermostat correctly. It's a 4 wire heating/cooling set-up... I was told maybe the transformer went out in the furnace, so I broke out the multimeter and I still got 24V on the secondary side of the transformer... any ideas??

Trace the 24 volts until you find where the problem is.