"itunes cannot run because some of its required files are missing. Please reinstall itunes."

I have been an iTunes library user for years and have around 4500 songs on file. Suddenly and without warning, the application has disappeared from my system. When I click on the iTunes icon to open the Library, I get a message saying:

"itunes cannot run because some of its required files are missing. Please reinstall itunes."

However, there is no instruction on what to do to put this right. Why can't the system simply show me which files are missing and supply a button to click on and restore them? I don't want to risk reinstalling iTunes, as I would probably lose my entire Library in doing so. I am visually disabled and not particularly Computer-friendly and so would welcome suggestions on a quick and easy way to restore my Library, if possible, please? Being partially sighted, my iTunes Library plays a major role in my life and losing it would be a nightmare.