As Don mentioned, the two common causes for a frozen evaporator coil are an undercharge of refrigerant and reduced airflow.
If only a portion of the evaporator coil is frozen, chances are that there is an undercharge of refrigerant. However, if the entire coil is frozen, chances are you have restricted airflow through the coil. What follows are some reasons/explanations for the partial/full coil freezing scenarios:
If the system is undercharged, the operating pressures and temperatures in the evaporator will be lower than desired. This means that the liquid refrigerant in the evaporator coil will actually boil at a temperature that is lower than 32 degrees F, causing ice to form on the coil. Now, because there is a deficiency of refrigerant in the evaporator, the amount of liquid enterng the coil is reduced, so the liquid boils into a vapor faster. Once all of the liquid refrigerant boils, the temperature of the vapor in the evaporator rises (superheat). In this scenario, the warm cooling coil cannot adequately cool the space.
Now, if the airflow through the coil is restricted, there is no shortage of refrigerant in the coil. What happens now is this... In order for the liquid refrigerant to boil in the cooling coil, we have to add heat to the refrigerant. The source of this heat is the air that passes through the evaporator coil. With a restricted airflow, the amount of air is reduced and, therefore, so is the amount of heat that is available to boil the refrigerant. With a reduction in heat, the operating pressures and temperatures of the evaporator will drop, bringing the coil temperature to a point below 32 degrees, once again causing frost/ice to form. Since there is no deficiency of refrigerant, the entire coil will freeze.
If you understand the concept of evaporator superheat, the following will prove invaluable. A low evaporator temperature and pressure can indicate either an airflow problem or a refrigerant deficiency on the low side of the system. The evaporator superheat will help us determne which scenario is the correct one. High superheat is an indication of an underfed evaproator coil, which is indicative of a refrigerant undercharge. Low superheat, which is an indication of an overfed evaporator, will be present when an airflow problem exists.
Here are some concepts that you might find useful:
Low evaporator pressure is an indication that the evaporator might have a deficiency of refrigerant.
High evaporator superheat is an indication that the evaporator might have a deficiency of refrigerant.
High evaporator pressure is an indication that the evaporator might have an excess of refrigerant.
Low evaporator superheat is an indication that the evaporator might have an excess of refrigerant.
Therefore, if the evaporator pressure is low and the superheat is high, the evaporator has a deficiency of refrigerant.
Also, if the evaporator pressure is high and the superheat is low, the evaporator has an excess of refrigerant.
Now the cool part... if the evaporator pressure is low (an indication of an uncharge) and the superheat is low (an indication of an overcharge), there is an airflow problem thorugh the evaporator.
Of course other factors need to be considered, such as the type of metering device in the system.
Hope this helps!