Why isn't my freezer getting cold?
Ice buildup on the evaporator is a sign of a low refrigerant charge. To verify this is the problem I would defrost the unit first, it could also be a bad defrost timer or a fan that died or even bad door seals. Since we don’t know the model or type all are guesses but these are the common problems. 

If you defrost the coil and it immediately comes back (I mean in a couple of hours) low refrigerant charge is the most common issue. If after defrosting it takes several days to ice up I would be checking the door seals, fan and defrost timer. 

I have found newer fridge/freezers do not last like they used to. I have found adding a service port and recharging brings the unit back to life for a few more years--I almost never find any leaks but believe there must be some that are smaller than my leak checkers can detect.  With these once the problem occurs I have found in 3-5 more years they do it again.  The lucky thing with r134a I have found is the charge is quite small many times less than a pound so it is easily fixed or topped off.