Could be a fill valve, faulty flapper, or there could be a pipe that is broken.
Anything but eggshells, Rhine's, Bones, grease, etc.
If the system begins to go low on refrigerant the unit will have little to no air flow and ice accumulation on the lower line on the outdoor unit.
Reset the power. If the issue reoccurs professional service may be needed.
Usually that means the drain is clogged. If the drain is not clogged, professional service may be needed.