A frozen evaporator coil is ice on the indoor coil that absorbs heat from your air. It is a symptom, not a disease — and the two things that cause it are low refrigerant and restricted airflow. When refrigerant is low (a leak), the coil runs colder than designed and condensation freezes on it. When airflow drops — a dirty filter, a failing blower, blocked returns — the coil also gets too cold and ices over.
Once iced, the coil stops cooling, and the ice blocks airflow further. Safety switches shut the compressor down; the ice melts; the system restarts and freezes again — a worsening short-cycle that wastes energy, leaves the house humid, and, if the compressor runs against the blockage, can pull damaging liquid refrigerant back into it.
The right response is to thaw the coil and then diagnose why it froze. If it is airflow, the fix may be as cheap as a filter or a blower repair. If it is low refrigerant, that means a leak to be found and sealed — and if the leak is in the evaporator coil itself, replacement runs roughly $800 to $2,500. A provider who just scrapes the ice off and adds refrigerant without finding the cause has treated the symptom and left the real problem in place.