Missing songs in iTunes show up with an exclamation mark next to the track listing. These are songs or videos that iTunes cannot locate, as the track files were moved or deleted from the indexed location.
iTunes can, therefore, not play these tracks. You can double-click on a missing song or video in iTunes to try and locate it.
If you have the songs loaded on an iPhone, iPod or iPad, the easiest way to bring back and fix your missing iTunes tracks is by using CopyTrans and its Smart backup feature. Here’s how it works: How to transfer music and videos from iPod to iTunes