Too much water? Potted hibiscus need to dry out before you water again.

Too small a pot? It's hard to tell how big it is from your pic, but I'd have a plant that size in at least a gallon pot.

Just my best guess.

Did you just pot this up? What variety? I think it may be in shock. Cut off the top foot or so, and keep in bright light, watering when soil starts to get dry. Do NOT let it get too dry, or be waterlogged, either.