yellow leaves and one dead plant..help?

i had 3 plants now i have 2,yesterday i saw that all the leaves became yellow till 20cm in the edge
and on of them all the leavves.
i grow them in a box size 43X60cm,the high is 130cm,i use in 250w hps in 12/12 the tempeture is 30c.
i used organic fertilizer until now everything was ok
why this is happens??

Temps are a little high. And those pots look awfully small for plants of that size. You might want to consider transplanting, or your yield will be shit if you even make it that far. I would have a plant that size in a 5 gal bucket at the least.

I agree , you should have re potted weeks ago. Put it in a much bigger pot, in a week it will be better than ever and in flowering it will triple in size.