More than likely your defrost heater coil has failed. It could be the defrost timer which sends power to the defrost coil once or twice a day (I can't remember which).
If your unit is a top freezer type, a quick , but not permanant fix is to empty the freezer and place a large metal pot of (hot to touch) water inside to melt the ice. This will buy a few days of cooling.
To check your defrost timer you must first locate it. Could be under the unit or mounted somewhere inside on cooling side. It will have a small 1/4" knob looking thing located on it. If this turns very very (slow slow you can't tell it's turning) slowly then at least you know the clock portion of the timer is functioning.
To replace the defrost coil just pay close attention to how the puzzle of plastic comes apart. The new coil is pretty much a plug and play device. It can be a little fussy removing the old coil from some of the coils. They are almost laced onto the cooling coil. Becareful handleing the coil. If you puncture it your day will be ruined. It is doable.
defrost problem, most likely defrost terminator is bad. this is a small thermal disc mounted somewhere on the evap coil. to be sure you need to check all three parts of the defrost system. 1) it will either have a defrost timer or an adaptive defrost circuit board 2) defrost terminator 3) defrost heater. The terminator and heater can be ohmed to check them.