HVAC Tech here.

As far as basic problems, lack of air flow is a good possibility. As a few others stated, your filter may just be dirty. Even worse, your evap. coil (pictured) may be dirty and/or clogged. This lack of air flow can also happen if your indoor fan is failing or dead, Your outdoor unit will bring in high pressure cold refrigerant, and if it doesn't have the means to transfer that cold temperature and pick up heat from the indoor air, it will freeze up. Worse case scenario, you are low on refrigerant, or have a restriction of refrigerant flow.

So...

Turn unit off, and let the evap. coil thaw out. (obviously) Check and/or replace the dirty filter. Check and/or clean the evap coil. (they sell coil cleaner, but this can get a bit messy.

Tomorrow, in the heat of the day go outside, and check your two refrigerant lines going into the house. The larger one should be fairly cold and if ambient temperature permits, sweating. The smaller one should be fairly hot, to the point were it is uncomfortable to keep your hand on it. If the lines are not quite cold and hot enough, your unit will not be transferring heat efficiently enough to prevent your evap. coil from freezing up during colder times (at night). A technician may have to come out and add refrigerant/ fix a possible leak, or find a refrigerant restriction, most likely in the orifices or thermal expansion valve.

Good luck.