"Frost Free" freezers have air channels and drainage pipes in the back concealed behind the lining, where the frost is accumulated, then melted and drained away. If the door is left open with the compressor running, the frost load will be too great for it to cope with.
It is quite common for these to become choked with frost and ice, after which the freezer will not work properly. Once clogged, it will no longer defrost itself, nor will it chill properly.
Unplug it (so the compressor is no longer running), empty out all the food and eat it, and leave the door open for 24 hours. You will probably find a surprising amount of water dribbles out into the cabinet, and into the drainage dish at the back (if you pull the freezer away from the wall you can see this drainage dish and swab it out to remove mould, grime and so that excess water does not drip onto the electrical parts).
When it has stoped dripping, mop it inside and out with paper towels so you are sure it is dry, leave it another hour to make sure there are no more drips inside or out, then close the door and plug it back in.
Knowing how to remove frost blockages can save you the expense of buying a new freezer. It is absolutely essential that you leave it long enough for all the interior frost and ice to melt and dry out.
