Frost building up inside the freezer usually indicates a problem with the refrigerator's self-defroster. This system gently heats the freezer compartment to melt the frost and drains any water into a pan under the freezer. If this system isn't working, frost builds up in the freezer compartment and on the evaporator coils.
A freezer with built-up frost isn't just annoying; the frost is also likely to stop the freezer from functioning correctly. When frost builds up on the evaporator coils, they don't get the air flow they need to work correctly. Eventually, the freezer stops cooling entirely and food can spoil. A manual defrost will work temporarily but won't correct the root problem.
Manually defrost the freezer by removing all food and turning the unit off for one to two days. If the freezer works properly after this, look at the defrost heater -- a component hidden behind the freezer's interior panel. If it looks damaged, it may need to be replaced. Problems in the defrost thermostat and timer can also cause excessive frost but are hard for homeowners to identify. If you suspect these problems, call a repair technician.