“out of the blue”
if something happens out of the blue, it happens suddenly and you are not expecting it
به صورت غیر منتظره، ناگهانی
Example 1: Then one day, completely out of the blue, I had a letter from her.
Example 2: a long-unseen friend who appeared out of the blue
Example 3: I haven’t seen them for years and then they call me out of the blue.