all that happened in one day? couldn't be ferts, it would not have happened that quick, same with overwatering, those leaves are shrivelled up, not droopy(they are, but not like you see with overwatering), tell us what pesticide you used(so we know what to avoid buying) and for god sakes throw it out! (or try it on something in your garden, like a flower, or a dandelion if the effect is that quick, it should poison anything with similar result then you'll know for sure).
Sorry for your problems, maybe it'll bounce back...