Q: I recently had to have my computers operating system wiped out and reinstalled.  He thought he saved all music and it does look like its there but when I click on the song in iTunes it says it could not be used cause the original file cant be found. Help

What are the iTunes library files? - http://support.apple.com/kb/HT1660

More on iTunes library files and what they do - http://en.wikipedia.org/wiki/ITunes#Media_management

What are all those iTunes files? - http://www.macworld.com/article/139974/2009/04/itunes_files.html

iTunes 9 [and later]: Understanding iTunes Media Organization - http://support.apple.com/kb/ht3847

Image of iTunes folder structure Dec. 2011 - https://discussions.apple.com/message/16923545
see also:
Image of folder structure and explanation of different iTunes versions (turingtest2 post) - https://discussions.apple.com/message/13025536 and https://discussions.apple.com/message/17457605

Now try to play a file.  When it asks you to find it, go to the location (based upon the structure you have now learned from the above links) and locate it.  Hopefully itunes will also ask you if you want to locate others based upon the same structure and it will slowly find the others.

Okay.  Read my words.

There's iTunes the program.  This is what opens your library.

There's iTunes the database.  This contains the thing that tells iTunes the program what music you have and where the  files are.

There's your media files.  When you click on a track in the iTunes program it goes to the database which tells the program where the music file is located on your computer that it needs to play.

Your database is broken.  It does not know the correct location for your files.

The program and your "library" (which is your database+media files) are two different things.  You can wipe the program from your computer and the library will still be there.  You can wipe the library files and iTunes will still open (to a blank library).

Your operating system on your computer is what runs the computer.  It has nothing to do with iTunes other than maybe some of the program components might be stored there.  Re-installing iTunes should make it all work again as long as nothing was done to the iTunes library files. However, since iTunes the program is opening, in your case something was done to those library files.  Only the person who saved your music knows the current situation (unless you look at the links above and learn a bit about how iTunes works so you can check yourself).  Maybe they saved the media files but did not save the database.  Maybe they moved your media files to a different location and the database is still looking in the old location.  We can't tell which from here.

Re-installing iTunes the program will not reconnect all this if the location information is incorrect.

As for your phone, we don't know if your music files or your library files are missing from the computer.  It might still all be there, just not in the right places.  You can transfer items purchased from iTunes from your phone to your computer.  If you got them from some other source then you have to use special third party software to do this.  We still don't know if you even need to do this since it might still all be on your computer.  Only you can tell us learning a bit how iTunes works.  Or take it into a computer store (not the one you took it to before since they made a mess of your iTunes when re-installing the operating system).

Oh, if by "re-installing the operating system" you meant your drive crashed and had to be wiped then of course all your other files are missing, but you haven't said that.  You can re-install an operating system on a computer and the data files should remain intact.