First check the filter, if its dirty change it. For now though change your thermostat to cool / off . Either let it sit and defrost itself or put it on fan only and it will defrost.
When things settle down you need to check or have someone check your a/c coil ( its called an A coil, that's the shape of the coil.)
If its clogged that will cause the same problem of frosting up. Most of the time its the filter.
Look at the outside unit, if its real dirty that could cause it. The fan in your inside unit may be clogged with dirt and that lowers the airflow, that will cause it.
Last thing is low gas pressure, Low Freon will cause this problem. If you check all the others and they are good, make sure whoever checks the pressures really knows what they are doing.
Filter, Fan, A Coil, Outside unit, then pressure. Hope its the filter. Cut off the A/C and it will defrost.