Change the battery in the thermostat on your wall. This may sound foolish, but even new batteries can sometimes malfunction. If the heat pump begins to work, you have just solved your problem.
Test the override switch. Unsnap the cover of your heat pump thermostat. It should just pull off by a couple of plastic snaps on either the top or the bottom. Underneath the cover, there will be an override switch. This directly bypasses all the electrics in the thermostat proper, and when you push this, it will manually complete the circuit for the heat pump to run. If the heat pump runs when you push the override switch, the thermostat is bad and needs to be replaced. If it doesn't run when pushed, you will have to proceed to the actual unit itself, which will be located outside of your house.
Take the cover off the outside unit. At the unit outside, you will have to remove the cover, so using your screwdriver, find the screws that hold it in place, most likely located near the ground, and there will probably be about eight to a dozen of them.Turn them counterclockwise to get them off. Pull the cover straight up.
Clean the heat pump coils. Use your whisk broom to gently brush off the coils and remove any debris from around them.
Turn the unit on to see if there is any difference in performance. If the coils were clogged with refuse, they virtually were unable to work at all. By cleaning the coils, you may have solved any problems you had with your unit.
Check to see if the heating/cooling fan is spinning by turning on the York heat pump unit. If it is not spinning, that means the bearings on the motor are bad, and it will need to be replaced. You absolutely need airflow over the coils to make your heat pump work, so if this is not happening because your fan no longer works, a professional will have to come in and change it out. If the fan works and the coils are clear of refuse, but your heat pump still does not function properly, the compressor is bad, and a professional will need to be called in to replace it. However, you will have eliminated several less significant options before calling in the repairman.