What are natural and correct words to express these situations?
When I was taking a class, I didn't pay attention to the teacher's speaking.
There are two situations when I didn't pay a attention to others' talking
1. My mind was blank and I didn't think about anything.
2.I was thinking about other thing. For example, when the teacher was talking about business, I was thinking what dishes I gonna to eat.
How do you say these mental status in daily life? I wanna know how to express it naturally, thanks! :)

Question

What are natural and correct words to express these situations?
When I was taking a class, I didn't pay attention to the teacher's speaking. 
There are two situations when I didn't pay a attention to others' talking
1. My mind was blank and I didn't think about anything. 
2.I was thinking about other thing. For example, when the teacher was talking about business, I was thinking what dishes I gonna to eat.
How do you say these mental status in daily life? I wanna know how to express it naturally, thanks! :)

Answer

1. A blank mind is accurate here.  Maybe you could say you were not focused.

2. This is called daydreaming.

Answer

1. My mind was blank and I didn't think about anything.

One might write::  "I was distracted."  "I  was not interested in the lesson."
"I felt indifferent."  "Mentally,  I was in another place entirely."  "I felt abstracted."

--------------------------

"I was thinking about other thing. For example, when the teacher was talking about business, I was thinking what dishes I gonna to eat."

First,  do not use   "gonna" in written communications. 
 The proper expression is  "going to eat".

One might write:

"My mind was preoccupied with thoughts of food."
"I was hungry and could not concentrate at all."
"The subject of business could not hold my  interest at all."