welcome to GoY, Sorry but lots of questions to help you with this problem ;o)
is the plant waterlogged?
next question: is it in a pot or in the ground? If in a pot how long has it been in the pot and when did you last feed it? The lighter green suggests it needs a good feed. as a short term remedy give it a dose of Epsom salts. half a teaspoon to 2 pints of water. If it doesn't perk up in a couple of weeks repot it [if in a pot] and check the roots for any maggots/grubs that might be feeding on them.