I had a new thermostat installed in December 2004 - the heating has worked all winter. Now the central air does not work. The fan also does not work (switch from auto to on doesn't do anything.) The thermostat is a Honeywell CT3200, configuration as follows:

R - Red
Rc - Orange
W - White
Y - Black
G - Blue

None of the wires are labelled, and my previous thermostat was also a 5-wire system (Rh, Rc, W, Y, G). The AC worked fine with it.

Unfortunately I was not around at the time of wiring, so I don't know where the wires were in the previous thermostat, or if they're in the same configuration again.

Any help would be appreciated.

The furnace should have about the same terminals. What you need to do is to have the same wires connected the same terminals at the thermostat and furnace. You wire colors are nt the standard ones. The black wire needs to run from the Y terminal on the thermostat to the Y terminal on the furnace, and then to the coil of the A/C.

This is the standard set up. Adapt your wire colors to it.
Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.

Well good news... my neighbour was getting AC installed today and she slept in, leaving the poor HVAC guys stranded on her driveway for awhile. They agreed to come in and fix me up at no charge, so it's all good now!

Black and white wires were swapped out!