It is difficult to predict the cause of the burning smell and the failure of the cooling in your freezer without more details. The smell could have been caused by an overheated fan motor or compressor. If you cleaned a large amount of dust from under the refrigerator, some of the dust could have caused the burning smell.
If you removed the bottom cardboard cover on the outside back of the refrigerator to clean out dust, make sure that this cover is properly replaced. The freezer will not cool properly without this cardboard cover in place.
Check the back wall of the freezer for excessive frost or ice. This could indicate an automatic defrost system problem. If you see this, you may be able to temporarily regain cooling by emptying the freezer and refrigerator and then unplugging the refrigerator and allowing the unit to completely defrost with the doors open for about 24 hours. After restarting the refrigerator, it may resume normal cooling until you can get a service technician to diagnose and repair it.
A number of other failures could be causing your cooling problem in the freezer. A compressor problem or lack of Freon could prevent the freezer from cooling properly. An evaporator or condenser fan problem could also keep the freezer from working properly. I recommend calling a service technician to diagnose and fix the refrigerator.
If you want to attempt to repair the refrigerator yourself, resubmit your question with additional details.