Unable to locate music files

I've been using my ITunes for years. My stereo is hooked up to my PC and all I ever did was open ITunes, click on a playlist and enjoy. I recently got an Iphone, after I hooked it up..... I Tunes can not find any of the music on my playlists!!! Nothing has changed.....I can not for the life of me figure out why now....it can't locate music files to play like it has for the past 7 years!!! PLEASE help! I have even uninstalled and reinstalled ITunes just in case that helped...nothing helps...I can't play my music now.