Q: Can't play my itunes songs on my computer

I recently went to play songs from my itunes on my computer and every time I click on a song I get a message that the song I have selected cannot be played because the original file cannot be found.  I still have my entire Itunes on my Ipod,  but it does not seem to do any good to use touch copy to try to get the song back into itunes.    I am not sure what the problem is or what I have done to create this situation.    All the songs show up as being in my library just as always but they will not play.   Any suggestion?     Also, I want to copy my itunes collection from my computer to my new computer but I am having a difficult time accomplishing that as well.   Obviously I am not very tech savvy,  but I am not a total idiot either.    I am using windows on my computer, with windows xp on the computer that I now have Itunes on (that I can't get the songs to play from, and window 8 on my new computer.     Thanks.