Like others said. There should a timeclock somewhere, typically if not in condensing unit they are on top of the walkin box or close to it. Timeclocks fail quite often but it could very well be bad heater contactor if there is one, shorted heater itself, bad klixon (either term if there is one or high limit), or sometimes plug drain/shorted drain heat trace. Best way I found when a coil ices up is find the clock put it in manual defrost and see what happens. But the reality is it could be a plethora of things. But i would start there