The temperatures in the freezer vary from 0 degree, which is correct, to 40 degrees which is a little warm. I don't believe this will be a sealed system problem or the freezer would not reach 0 degrees.
The problem is more difficult because the temperature will vary in the freezer as the thermostat cycles the compressor on and off and the defrost systems cycles on and off. If the freezer is empty the temperature will vary and it would be normal.
Since the problem just started there are a couple of things I suggest you check. Behind the toe panel on the right hand side is the defrost timer. It can be turned manual clockwise to put the refrigerator into defrost. The defrost cycle should last about 20 minutes and you can time it. Sometimes the defrost timer will stick and the unit will not come out of defrost.
I also recommend you pull the back cover off and check the condenser fan and the coils. Lint can build up on the condenser coils and affect the cooling. If can be cleaned with a vacuum with and extension nozzle or if a vacuum is not available then with a condenser brush.
I hope this information is helpful to you in solving your problem. If you have any additional questions then submit another question with additional details for further assistance.