You may be out of refrigerant. Most Carrier H.P.s have a low pressure switch which will not close if most of the refrig. has leaked out. Could also be a problem with the control board (assuming that you have 24 volts coming into the unit.)
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Thanks for your answer chuckbox2; it was very helpful.
thermostat is another possibilty, chuck is right about the rest of this answer. so you need a voltmeter to check power on the thermostat leads for 24 volts on the contactor. remove the leads to check for power. pull disconnect or shut off breaker to unit first! this stops 240 volt power to the unit, but leaves the 24 volt circuit active. another clue about 24 volt circuit is to turn the fan switch on the t-stat to on or manual to see if the fan in the furnace runs.
