How to stop Central Air unit from freezing over inside and outside
There are a few causes for this, and the most common are in two categories - bad air flow and low refrigerant.

Bad airflow can be caused by a dirty filter, clogged ducts, or a poorly performing fan.  If the (relatively) warm air from your house isn't blowing over the coils, the condensation they normally create starts to freeze.  Once they start to freeze, airflow is reduced, the coils get colder, and even more ice forms. 

The other common problem is something a technician will have to check.  It sounds counterintuitive, but low refrigerant can end up making the coils too cold.  The results are the same runaway reaction described above - lots of ice, reduced airflow, and a compressor that never stops because the house isn't getting cooler.

Once the system thaws, check all your filters and make sure the blower fan works fine.  You can check the coil temperatures with an IR thermometer while the system is running.  If it gets below freezing and airflow seems good, it's probably time to call a tech.