On the inside of the refrig in the freezer compartment there will be a panel that can be removed. Once removed, you will see the evaporator coils. If they are caked with ice, that is the problem. They are supposed to be ice-free, where the accumulated frost is melted during the defrost cycle. If the coils are solid with ice, the system can no longer cool. There are three possible problems, listed here in order of likelihood:
1 Defrost thermostat has failed. This is a ten dollar part you can replace yourself easily.2 Defrost heater has failed. This part is around $15-20 on the internet, plus shipping. It is easy to replace.
3 Defrost timer has failed. This is more complicated and depends a LOT on the design of the refrigerator.You can learn all about this by going to google with the following search string :
Kenmore refrigerator doesn't cool.
There will be links there describing possible solutions, videos showing how to do the repair, and sources for buying the parts......
It ain't rocket surgery, but it does take a bit of self-education......