Answer: Based on your description I am not exactly sure what is wrong with your plant, but there are several possibilities. One is that some of the stems were bruised in shipping or during repotting and this has caused the leaves to shrivel. If this is what happened, they should regrow. In any case, remove any dried foliage in case it is caused by a pest of some kind. As a precaution, examine the plant very carefully for any signs of insect activity, too, checking the undersides of the leaves and the stems.
Mint needs a soil that is average to moist, so do not keep it sopping wet. Just keep it damp like a wrung out sponge. When you water, empty any excess out of the saucer. Newly repotted plants can easily suffer from overwatering, so be careful with this. Plants also use less water in winter when they are not growing as actively as they do in summer, they also use less indoors than they do outdoors where it is breezy.
Also, it's best to repot into only the next larger size pot rather than one that is substantially larger. It's best too to match the soil mix to the original. Sometimes a mismatch can cause either over or watering by either wicking away or holding excess moisture at the roots. Your plant might be suffering from underwatering, but this is less likely.
Another possible problem is that the humidity level in your room is very low. If you suspect this is the case, set the plant on a tray of pebbles in water and keep the pebbles wet. Add a few drops of bleach to the water to stop algae and make sure the bottom of the pot is above the water level. As the water evaporates it will humidify the immediate area around the plant. This is especially important if the plant is in a draft from a heating vent or near a radiator.
It is also possible that the window sill is too cold and the plant has been chilled, but I doubt that's the case in a modern building.
Mint grows in full sun to part shade outdoors, so the more light you can give your plant the better, especially in the winter. You might try supplementing the winter daylight with a regular light bulb, a pair of fluorescent bulbs (one warm and one cool), or possibly a gro-bulb.
Based on your description, I suspect the plant is overwatered and is also adjusting to its new growing conditions -- remember that it came out of ideal conditions and will need to adapt to the less than ideal setting it has now. With a little patience and a little luck it may pull through.
Q&A Library Searching Tips