The compressor runs but the inside is warm.
A stalled evaporator fan, frost blocking airflow, or lost refrigerant.
Defrost 24 h and retest. If it recurs, it's the defrost system. If refrigerant is low, we stop: that requires an EPA 608 license in the US.
It depends on the model and on what we find inside, so we quote each repair individually. The diagnosis fee depends on the device, and you approve the exact repair price before any repair starts. If it can't be fixed, you only pay the diagnosis.