Find the timer and advance it till it click into defrost. Listen for the usually defrost noises.
If it does nothing then the heater and termination thermostat are suspect. Do a visual of the evaporation area to confirm it then thaw out the ice box gently. Do go in there chipping away at ice or you will knock a hole into a freon line. Use a hair dryer or a garment steamer as some have recommended in the past. Then check for continuity on the heater. To check continuity on the termination thermostat you must have it in a freezer and get it to a blissful 25F before it has continuity. Warmth of your hand can get it to 50f rapidly so when checking that bad boy time is of the essence. Drop it in some ice water when removing it from the freezer to keep it cold while you do the check. Some folks swap both the heater and thermostat depending on how hard it is to get to the heater. Some change out the timer heater and thermostat so they can be done with defrost problems for a while. The bimetal defrost thermostat kills the power to the heater when the crawlspace temp get warm enough to melt frost off the coils where no ice can build up. It is the most likely problem.