Accusative. "Bread" is not animate, and for masculine non animate nouns their Accusative is the same as Nominative.
Having said that, there are quite a lot of nouns that are grammatically 'animate' for no reason, including most vegetables and fruits. But you will encounter it later, let's leave that aside now.
"Masz chleb" is more accusing or saying someone has bread. "Czy masz chleb?" should be the correct answer for "Do you have bread?" As "czy" would indicate a question instead of saying someone has bread. "Masz chleb?" Can work as a normal sentence asking if someone has bread, however it's slangier than should be taught I think.