In my experience, the biggest culprit causing these symptoms is the defrost heater. This is a heating device that melts frost off the "evaporator coils" better known as the coils usually located inside the freezer area of the unit. These coils get very cold and warm air that is let inside when any door is open, will carry moisture with it that will accumulate like water on the outside of a cold glass in summer time, only it accumulates on the evaporator coils and turns to frost.
As your fridge cycles through its operating stages, one part of the cycle turns off the cooling and turns on a heating device of varying types to melt the frost and ice off of the evaporator coils so it can drip down and flow through a small hose through an opening in the bottom of the fridge and into a pan underneath the fridge where it evaporates back into the air again.
If the heating device never comes on, or becomes inoperative, the frost will slowly continue to collect on the evaporator coils until it becomes a solid block of ice. This now greatly reduces the ability of the evaporator fan to blow the cold air around the coils throughout the freezer and the rest of the fridge. The ice built up around the evaporator coils will still provide some cold, but will significantly prevent the fridge from operating properly.
Another cause may be the evaporator fan not operating correctly. If it doesn't blow air over the evaporator coils well enough, cold air will not get distributed through out the fridge. Cold air tends to "fall", so the bottom of the freezer will be the coldest. Generally, there is an adjustable hole in "side by side" units so that the amount of air coming through that hole can make the "meat crisper" colder. In fridges where the freezer is on the top, I've seen the evaporator coils (which often incorporate "fins" like those on a car radiator, turn into a solid block of ice.
Also check for dust collecting on the condenser coils on the outside of the fridge. These dissipate heat drawn from inside the fridge and take it outside the fridge. These are often located on the back of the fridge or underneath the fridge. Dust and fluff act as insulation and degrade their ability to work properly.
Obviously, to repair the fridge, you should relocate the contents of your unit until you can repair the fridge. Fixing your fridge will take at least a few hours even if you can get your hands on repair parts quickly. This will involve unplugging the unit and locating the items I've referred to. Either allow the accumulated ice to melt naturally or GENTLY speed up the process with a hair dryer or heat gun. Do not use any force removing the accumulated ice and frost, you could easily damage the coils and make things much worse than they already are. Even to the point of making the unit uneconomical to repair.
Without your posting a make, model, type of freezer/fridge arrangement or other details, it is difficult to assist you further. Part placement and component locations vary greatly in this appliance.