A vague question really, but indoor or outdoor its generally lack of water. Sounds daft, but it can be too much water as well! Outdoor, I bet on lack of water. This could be from months ago, not just from recent dryness. Sometimes its caused from a lack of water from the previous year. I bet you are talking about an evergreen too. More info, better opinion.
Your question title made me think. Should it have said Impatients? This is a "Busy Lizzie". If grown indoors it can be caused by to hot and dry atmosphere, and again by over or under watering. Dip your finger in the soil, is it very wet? If it is, repotting in fresh compost is best.