用css和中心文本叠加图像上的颜色

I am trying to make it so when users hover over an item that is no longer in stock it will show them a message overlay on the image saying no longer in stock. This is working currently but I cannot seem the get the text to float in the absolute center of the block. It floats on the top in the center of the image. I am trying to make it so when users hover over