Maytag top freezer model mtb2156bew how can I repair

You will need a meter to do some tests. First, see if the freezer fan runs when you depress the door switch. If so, you want to clean the coils to eliminate that as a cause of any problems. Clean them with soap and water by unplugging unit and removing front kick plate and rear panel. Then spray coils down with spray cleaner and wash off with sprayer on water hose with small amount of water and vacuum up with wet-vac. Let refrigerator stay off an hour or two in case compressor has overheated. If freezer is iced up defrost it by opening door. You should pull out condensate pan from front if it's located there and clean it, too. After this is done you will be ready for next steps.
After it's clean locate the defrost timer and turn it until it clicks and turns off compressor. Wait and see if the refrigerator comes back on in 15-20 minutes. The freezer fan and the compressor will turn off. If it comes back on then remove the screws holding on cover to freezer coils. Most of the time they are in the floor of the freezer, so you will have to remove two Phillips-head screws, one on each side, and slide out cover. Carefully remove Styrofoam cover on top so you can see all around coils. You will see an aluminum heater wrapped around the evaporator coils and one wire will go through a defrost thermostat. Unplug that heater and check it with an ohm meter unless you can plainly see it is burned in two. Most are burned in two when bad. If good you will read about 300 or so ohms, if bad it will read infinity. If it is bad carefully remove it and do not bend the tubing sharply or the capillary tubing as it's very delicate. You may have to lift the coils up a little to replace the heater and the t-stat. You should replace both if the defrost heater is bad.
If the compressor does not run at all you most likely have a bad thermostat and you can get a universal replacement at any parts store. Just make sure that the amount of cap tube exposed is the same as the old one and in the same place. Here are the manuals: https://dl.dropbox.com/u/8010163/Maytag%20Refrigerators.zip
How to use a meter: https://dl.dropbox.com/u/8010163/Whirlpool%204322537A%20How%20to%20use%20Test%20Equ ipment%20Service%20Manual.pdf