Good question. I don't know how familiar you are with working on PCs but don't hesitate to tell me if I'm being patronising or too techie.
Firstly, find where (which folder and which drive) you store your mp3 files, physically - lets call it your library. If you've also created any playlists that relate to them, then locate them too. You won't need to take back up copies of the mp3 music files (tho that's a pretty good thing to do from time to time anyway) but I would recomend taking a copy of your playlists into another folder/another computer/memory stick. Also save anything else you might have created in there such as notes to yourself about stuff you've thought, music images, lyrics or whatever.
Then do the un-instal and re-instal.
Check that both QuickTime and ITunes works with any tune - any mp3 file from anywhere.
That lets us know we've fixed the problem!
Only if that's okay, then start up ITunes and get into setting up your preferences and options, which range from how things appear on screen to what language you use and more. Find where you have to specify where your music libary is stored and a) make it point to your library (you noted that folder, above), then b) get the application (ITunes) to search for content and re-build your library index. Lastly. c) copy back your saved playlists.
If you need more assistance then your going to have to supply a LOT more nfo - such as your OS, and directory structure, applications associated with sound devices and music file-formats, and upgrage history. I may not be able to help
