If this has happened to all your files it because you've moved them (renamed directory containing music) or your iTunes library is corrupt.
If you moved your files you can use something like dreamweaver to do a search and replace on the XML library file and update the worng path to each file with the correct one.
If you have a Mac, you can use tool that automatically fixes this by Spotlighting lost files: http://itunes.pornel.net/
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To drag a music or video file into your iTunes library, left-click the file and hold, drag the file into your iTunes library and it will sync the file into your library.
Copy the file/folder you have downloaded into 'My Music' (found within 'My Documents') Once the music has copied got to itunes and under file select 'Add folder to Library'. Find the folder you just added and select it. itunes will do the rest.
itunes error 13010 refers to a corrupt itunes library genius file -- to resolve it remove the "itunes library genius.itdb" from your itunes folder and relaunch iTunes
You get songs from mp3skull to iTunes by finding the song you want to and right clicking on download. It should ask you where to save the file change the name to whatever you want it to be called and find the iTunes library folder save it in there, it should then appear in iTunes. If not then press control-O and it will ask you to find the file you want to save to the iTunes library, find the file and press select, it should then appear in iTunes.
To put a song on your Apple device, you must first put it on iTunes. You can do that buy importing a mp3 file from your computer, or purchasing it from iTunes directly. Then you can sync iTunes with your device which copies the songs onto it.