i have a musa acuminate banana plant with wilting leaves

i live in ontario canada near toronto and it is winter here. my plant was purchased from a local nursery and was in good shape when i bought it, however after 2-3 weeks it has begun to wilt. i do not know what i am doing wrong, it gets full sun, and i have followed the care instructions, i even mist the leaves with warm water. i am on the 18th floor of an apartment building and have a south facing window.