My poinsettia had been doing really well, but was moved for a couple of days to a different window ledge, and now the leaves are starting to fall off. The leaves initially began drooping, and then began spotting. The internet suggests that the plant is reacting to chemical fungicides; however, I have not used any on the plant. Is there any way to recover this?
Check the soil moisture. The soil should be moist, but never soggy.
Check the soil before watering, don't water on a schedule.
If the plant has been in the same soil and pot for some time; it may be time to repot the plant with some fresh, good quality potting soil.
Here are some links with more care information.