Why won't the song play?

I've been downloading some mp3 songs lately and had them transfered to
Itunes. But the problem seems to be, after a few minutes or days when
i want to listen to the downloaded mp3 songs, it says that the
original file could not be found. it shows this same sign to each of
the mp3 songs. this sucks because i can't listen to it,can't change
anything, and can ony be heard on my ipod. and when i want to locate
it, it goes to my software of some kind. =S
Is there a way to fix this problem? Is there a way to listen to the
song? If you know how, pleaseeee, please tell me because i so want to
know how to get rid of this stupid thing.
best one, best ratings. thanks =D