Do you mean you've deleted/cleared popups.ini? If not, you can just load it from the Script Editor again.
If you have lost that, you can type find copies of all of the file's mIRC uses by default in the defaults directory within mIRC's installation directory (ie. //run $+($nofile($mircexe),defaults) ). Just copy scripts\popups.ini from there to your regular settings location and load it from the Script Editor.