My guess and from past experience of this happening is it is being over watered or the plant is getting to cold. How often do you water and what type of temps are the plants exposed too? What light cycle and nutes? Looks like it can recover fairly easily once the problem is diagnosed.
Okay I see the twisting in the new growth. And I also agree with yoda as it may be a chlorine issue. If so those leaves may stay deformed through out the grow but all new growth should come in normal.