Songs from the playlist cannot be found

Hi! Who can help me with this problem:

Some songs in the song list show an exlamation mark in front of the title. When I click on them to play them I get the message: "The title"XY" cannot be played because the original could not be found. Do you want to search for it?" I tried several ways to search, but couldn't find the songs. How can the songs be on the list and still they cannot be played? I once loaded them up to iTunes.

Thank you!

Roland (Germany)