Refrigerator is not cooling, but freezer is working fine
The ice chunk is an indication of a low Freon charge, or a evaporator that has a restricted air flow. Some units only actively cool the freezer and use fans to move cold air to the reefer making sure these vents are open and clear may show the problem if the refer has its own coil I would expect a low Freon charge to be the problem. Most refredgerators don't have ports to recharge but one can be added and the Freon topped up. R134a is very common and r404/410 are other types that are common with a few oz added the system will work like new if there are no other problems.