Do you mean ice that fell from a snow storm while the 'main' power is off? or existing ice [frost] built while it was running and then lost power? Or, power is on but the unit is just turned off at the T-stat?

If powered on, but turned off at the t-stat the compressor will stay warm but it will not actively defrost until you call for heat - usually after a 5 minute on-time delay, followed by enough run time for all the sensors to determine if there is frost.

If the main power is off then the answer is NO - Make & model dependent, the outside condenser unit may have a compressor cartridge heater, crankcase heater, or a belly band heater. These heaters insure the oil stay liquified while the unit is powered on but not actively running, however if you lose power then those heaters will shut off, which could be cold enough for the compressor to locks itself out. Once power is retorted it can take a fair amount of time to warm back up.