Poinsettias are extremely sensitive to the cold, and exposing them to temperatures under 50 degrees Fahrenheit can kill them. If your plant loses all of its leaves then it is completely dead, but if there are leaves left try trimming off damaged areas and placing the plant in a bright spot with indirect light and temperatures between 65 and 75 degrees Fahrenheit with no drafts. Water it every few days to keep the soil moist, then take it outside once the weather turns warm. If your efforts have been successful, it will begin to send out new leaves in the summer.