Could be a nutrient lock out due to PH imbalance - Are you growing in soil or any other medium? How old are the plants? Are they still vegetating?
If it curls down like a claw and starts having a yellow tint, i would go for a Nitrogen deficiency. Not necessarily that you do not have Nitrogen in your medium (soil/compost should have enough of that) but its not getting absorbed because of incorrect PH range of the medium.
Pics should definitely help us getting to the correct diagnosis and also some more details on size of pot, medium, nutes used (if any) etc...the more details we have, the closer we can get to the solution.