If your evaporator coil freezes over, then you should not use your AC unit until it is defrosted. Be sure to turn off your unit — you can turn your thermostat to “Off” for the time being. Thawing the coil may take up to a day. You may have to keep an eye on water buildup in the drip pan, since it can overflow. Once it has thawed completely, you can turn your system on again.