How to get a close icon on popup using HTML/CSS style?

Question

Scenario: I have a text box. I also have a button. On clicking the button, I'm getting one pop-up. Whcih is not displaying the close(X) icon. I need close icon (X) on the pop-up.

I have a background image for the close icon (X) on the local path.

Please suggest CSS style for the same.