neden yok means "  there is no reason " .

neden olmasın ? means " Why not ?  "

neden yok? = why isn´t there any (of it)?

ah ... like saying " Ekmek neden yok ? " = why there isn´t any bread ????

I don´t know if that is right !!

Almost! You need to change the order in English when it´s a question. And order change in Turkish one makes some difference.

Ekmek neden yok ? = what is the specific reason that there isn´t any bread ?

Neden ekmek yok ?  = why isn´t there any bread ?

Niye ki means "why not" right?

Ekmek neden yok ? why

Neden ekmek yok ? bread

general rule, emphasis is on the closest word to the verb.

Neden yok, Nedeni yok, Sebep yok or Sebebi yok.  means that (as an answer ) there is no reason

an example of this usage:

A: Bugün neden sinemaya gitmiyorsun? Why dont you go to cinema today?

B: Nedeni yok. Sadece canım istemiyor. There is no reason. I just dont want to go.

Neden yok? means that  ( as a question ) why isnt there any ....?

A: Ekmek var mı? (do you have some bread? )

B: Hayır , yok. ( No, we have not )

A: Neden yok ? (why not? )

B: Ekmek az önce bitti.  ( It is just finished. )

Neden olmasın. (as an asnwer )  It is a way of saying that something is possible to do , or a way of saying yes.

A: Bu akşam yemeği dışarıda yemeye ne dersin? ( what about eating out tonight? )

B: Neden olmasın. ( why not. )

Neden olmasın? (as a question ) a question used to learn reason of something.

A: Yarın seni ziyaret edeceğim. ( i will visit you tomorrow )

B: Olmaz.  ( it is not possible )

A: Neden olmasın? ( why not? )

B: Yarın evde olmayacağım. ( i am not going to be at home tomorrow )

that was helpful! I got it, thank you.