It has nothing to do with 2D matrix. You may find the instruction in Find Minimum in Rotated Sorted Array.

Suppose a sorted array is rotated at some pivot unknown to you beforehand.
(i.e., [0 1 2 4 5 6 7] might become [4 5 6 7 0 1 2], and [4 5 6 7 0 1 2] is the rotated sorted one).