Diagnosis
The evaporator coils are frosting up. This happens when the air flowing over them is too cold, or the airflow is restricted. It triggers an auto-defrost cycle, but sometimes the frost wins.
How to Fix It
First, turn the unit off and let the ice melt completely (this can take hours). The main causes are: 1) A dirty air filter (clean it), 2) Room temperature below 65°F, or 3) The unit set to 'Continuous' mode in a cool room. After thawing, clean the filter, ensure the room is warmer, and use 'Dry' mode with a reasonable humidity target (like 50%).
If That Didn't Work
If icing recurs quickly in normal conditions (room >65°F, clean filter), the auto-defrost thermostat or timer might be faulty. The unit isn't cycling into its defrost mode often enough or long enough.
Heads Up
Never chip away at the ice with a tool. You'll almost certainly puncture the delicate copper refrigerant tubes, which totals the unit. Let it melt naturally with the power off, or use a fan to blow room air into the intake to speed it up.