Try holding down the option key when you double click on the app in the Applications folder.
Which version of iTunes are you running? You can still download older versions on the Apple website. A reinstall of the same version or a little higher should fix everything. Do not try switching to Music. That's a nearly irreversible step that can easily cause days and days of grief.
Before going to that extreme, I'd try playing with iTunes .itl files a bit. Sometimes they get corrupted. Fortunately, iTunes keeps backups. Inside your iTunes folder is a file called "iTunes Library.itl". There should also be a folder in there somewhere (it varies by version) called "Previous iTunes Libraries", which contains older .itl files.
Quit iTunes, if you've somehow got it running.
Drag "iTunes Library.itl" out onto the desktop, so you have a backup. Open your "Previous iTunes Libraries" folder and copy the newest .itl file out to where your "iTunes Library.itl" file was. Rename the file "iTunes Library.itl" and try starting iTunes.
If it doesn't work, drag the old .itl file off the desktop, and back to where it was. You're going to have to do a full OS reinstall, see link.