Oh, that is sad! Such a lovely plant.
The leaves are not curled or discoloured, so the most likely causes are cold and overwatering .
If it's been outside in the past few weeks, it could well be the cold nights we had. Several of my outdoor plants were affected; even lost some leaves off the beans.
Before you go all-out, changing the pot and soil, unblock the drainage holes. If the holes are blocked, extra water may be retained and coleus hates being waterlogged. Just stick your finger into the soil as deep as you can. If it feel moist, there's too much water. You may need to go manual, and give only it as much as it can use.
