cannot open ITUNES after updating program and adding icloud

I UPDATED MY ITUNES AND ADDED ICLOUD UPDATE . NOW I CANNOT OPEN MY ITUNE ICON IT TELLS ME TO DOWNLOAD ITUNES AND WHEN I DO IT WONT ALLOW ME TO CONTINUE DON'T UNDERSTAND WHAT IS GOING ON WITH THIS PROGRAM .I HAVE MANT DOWNLOADED SONGS IN MY ACCOUNT . I JUST WANT TO GET BACK INTO ITUNES.